You know those dinners that make you feel like a responsible adult and a rebel at the same time? This is that casserole. It’s rich, buttery grouper tucked into a creamy, cheesy sauce with a celery crunch that feels illegal on keto—but isn’t.
It’s weeknight simple, date-night worthy, and high-protein enough to make your gym coach nod in approval. No mysterious ingredients, no fake food—just legit flavor and a satisfying bite that won’t kick you out of ketosis.
Why You'll Love This Recipe
- Low-carb, high-protein gold: Grouper is lean, mild, and perfect for soaking up flavor without adding carbs.
- Creamy + crunchy combo: The celery brings snap, the sauce brings comfort, and the cheese ties it all together.
- 30–35 minutes total: Minimal prep, fast bake, and easy cleanup—your kind of dinner.
- Family-friendly: Mild fish and familiar flavors make it a win for picky eaters too.
- Keto without compromise: No thickeners or starches needed—just smart ingredients.
Shopping List – Ingredients
- 1.5 lb grouper fillets (skinless, cut into 2-inch pieces; sub: cod, halibut, or mahi-mahi)
- 2 cups celery, thinly sliced (about 4 large stalks)
- 1 small yellow onion, finely diced
- 2 cloves garlic, minced
- 3 tbsp unsalted butter
- 1/2 cup heavy cream
- 4 oz cream cheese, softened
- 1/2 cup mayonnaise (preferably avocado oil–based)
- 1 cup shredded Gruyère or Swiss (melts beautifully)
- 1/2 cup shredded Parmesan, divided
- 1 tbsp lemon juice + 1 tsp lemon zest
- 1 tsp Dijon mustard
- 1/2 tsp paprika (smoked or sweet)
- 1/4 tsp crushed red pepper flakes (optional, for a kick)
- 2 tbsp fresh parsley, chopped (plus more for garnish)
- Kosher salt & freshly ground black pepper, to taste
- 1 tbsp olive oil (for the pan)
- Optional low-carb topper: 1/3 cup crushed pork rinds mixed with 1 tbsp melted butter
Instructions
- Preheat and prep: Heat oven to 375°F (190°C). Grease a medium casserole dish (9×9 or similar) with olive oil or butter.
- Sauté the veg: In a large skillet, melt butter over medium heat.
Add onion and celery with a pinch of salt. Cook 6–8 minutes until softened and lightly translucent. Stir in garlic for 30 seconds.
- Build the sauce: Reduce heat to low.
Add cream cheese and stir until mostly melted. Pour in heavy cream and whisk to combine. Stir in mayonnaise, Gruyère, half the Parmesan, Dijon, paprika, red pepper flakes (if using), lemon juice, and zest.
Season with salt and pepper. You want a thick, spoonable sauce.
- Season the fish: Pat grouper dry. Season with salt and pepper on all sides.
Toss gently with parsley.
- Assemble: Add the seasoned grouper to the casserole dish in an even layer. Spoon the creamy celery mixture over the top, ensuring fish is mostly covered.
- Top it off: Sprinkle remaining Parmesan over the casserole. If using, mix pork rinds with melted butter and scatter on top for a crispy finish.
- Bake: Bake for 15–18 minutes, until fish flakes easily and the sauce is bubbling.
If you want extra color, broil for 2–3 minutes at the end. Don’t walk away—broilers are sneaky.
- Rest and serve: Let it stand 5 minutes to set. Garnish with more parsley and a quick squeeze of lemon.
Serve hot.
Storage Tips
- Fridge: Store leftovers in an airtight container for up to 2 days. The fish stays tender and the sauce rewarms nicely.
- Reheat: Warm gently at 300°F (150°C) for 10–12 minutes, or microwave in 30-second bursts. Don’t overheat or the fish can dry out.
- Freezer: Not ideal.
Creamy sauces can split and fish texture suffers. If you must, freeze up to 1 month and thaw overnight in the fridge before reheating low and slow.
What's Great About This
- Macros that work: High-quality protein, satisfying fats, and practically no carbs. Perfect for keto or low-carb lifestyles.
- Restaurant-level flavor: Gruyère + Parmesan + lemon + Dijon = the “how is this so good?” factor.
- Beginner-friendly: No browning the fish, no complicated steps, no culinary degree required.
- Celery’s glow-up: It’s not just a garnish.
It brings crunch, fiber, and freshness to a rich, creamy bake.
Common Mistakes to Avoid
- Using watery fish: If your grouper was frozen, pat it very dry. Excess moisture can make the sauce thin and the casserole soupy.
- Overbaking: Grouper cooks fast. Once it flakes with a fork, it’s done.
Dry fish is a crime against dinner.
- Skipping the salt: There’s cheese, yes, but you still need to season the fish and the sauce for full flavor.
- Too much lemon: Lemon brightens, but too much can break the creaminess or overpower the fish. Stick to the amounts listed.
- Heavy onion: Keep onions finely diced and well-cooked. Raw, chunky onion steals the show (not in a good way).
Mix It Up
- Protein swaps: Try cod, halibut, mahi-mahi, or even shrimp.
Adjust bake time for thinner fillets or smaller pieces.
- Cheese remix: Go Monterey Jack for extra melt, or sharp cheddar for punch. Keep Parmesan on top for the best browning.
- Herb upgrade: Add dill, tarragon, or chives for a more coastal vibe. Basil if you want a brighter finish.
- Veg variations: Toss in sautéed mushrooms or a handful of chopped spinach.
Keep it low-carb and avoid watery veg like zucchini unless pre-salted and drained.
- Heat level: Swap paprika for Cajun seasoning or add a pinch of cayenne. Your taste buds, your rules.
FAQ
Can I use canned fish?
Yes, but choose high-quality canned white fish or even canned salmon packed in water, drained well. Texture will be softer and the bake time shorter—aim for 10–12 minutes just to heat through and meld flavors.
Is grouper sustainable?
It depends on your region.
Look for wild-caught, hook-and-line options with a reputable certification. When in doubt, check local guides or swap for sustainably rated cod or mahi-mahi.
How can I thicken the sauce without flour?
The combo of cream cheese, heavy cream, and melting cheeses naturally thickens. If you want it thicker, simmer the sauce a minute longer before assembly, or add 1–2 tbsp grated Parmesan.
Does this taste fishy?
Not really.
Grouper is mild and slightly sweet. The lemon, Dijon, and cheese make it balanced and rich. If it tastes fishy, the fish may not be fresh—always buy from a trusted source, FYI.
Can I make it dairy-free?
You can try coconut cream and a dairy-free cream cheese alternative, plus a dairy-free melty cheese.
Flavor will shift a bit, but the method holds. Season generously to compensate.
What can I serve with it on keto?
Great sides include roasted asparagus, sautéed green beans with almonds, buttered broccolini, or a simple arugula salad with olive oil and lemon. Cauliflower mash also slaps, IMO.
How do I prevent curdling?
Keep heat moderate when building the sauce, add lemon at the end, and avoid boiling once the dairy goes in.
In the oven, the gentle bake keeps everything silky.
Can I prep it ahead?
Yes—assemble up to the point of baking and refrigerate for up to 8 hours. Add 5–7 extra minutes to the bake time if going in cold. Add pork rind topping right before the oven for max crunch.
Final Thoughts
This Keto Celery & Grouper Casserole hits the sweet spot: simple to make, big on flavor, and macro-friendly without pretending to be something it’s not.
It’s the kind of meal that converts “I don’t like fish” people and keeps keto folks happy and satisfied. Make it once, and it’ll sneak into your rotation on repeat. And when someone asks for the recipe?
Just smile and say it’s your weeknight superpower.
