Flounder Au Gratin

Tender flounder fillets baked under a silky Swiss cheese sauce built from a quick roux with scallions and diced celery. This is classic French-inspired cooking made approachable: a simple white sauce, good Swiss cheese, and mild fish that lets the gratin do the talking.

The sauce comes together fast on the stovetop. Sauté the scallions and celery in margarine until soft, stir in flour for one minute to cook out the raw taste, then gradually add skim milk while stirring constantly. The gradual addition is what prevents lumps; dumping all the milk in at once creates pockets of dry flour that never smooth out. Once the sauce thickens and bubbles, reduce heat and stir in the shredded Swiss until it melts into a smooth, glossy coating.

Pour the sauce evenly over the fillets arranged in a single layer. Cover and bake for 25 minutes. The fish is done when it flakes easily with a fork; flounder is thin and delicate, so check early. Overcooking turns it dry and stringy. A sprinkle of fresh minced parsley at the end brightens up the dish.

Pro Tips

• Thaw frozen fillets completely and pat dry with paper towels; excess moisture will thin the sauce
• Stir the roux constantly for the full minute before adding milk to prevent a floury taste
• White pepper instead of black keeps the sauce’s appearance clean and speck-free
• Use a baking dish just large enough for the fillets in a single layer so the sauce pools properly

Variations

• Gruyère: Swap Swiss for Gruyère for a nuttier, more complex flavor
• Herb-crusted: Top the sauce with seasoned breadcrumbs before baking for a crispy gratin crust
• Sole swap: Use sole fillets instead of flounder for an equally delicate result