If using frozen scallops, fully thaw in the refrigerator and pat dry with paper towels. Remove any side muscle if present. Season scallops lightly with 1 tsp salt and 1/2 tsp pepper.

Sear scallops briefly

Preheat a large skillet over medium-high heat. Add 2 tbsp unsalted butter. When butter foams, add bay scallops in a single layer and cook 1 to 1.5 minutes per side just until they are opaque and slightly golden — do not overcook. Transfer scallops to a bowl and reserve any flavorful browned bits in the pan.

Deglaze pan

With the skillet still hot, add 1 tbsp dry white wine and 1 tbsp lemon juice to the pan to deglaze, scraping up browned bits. Let reduce for 30–60 seconds, then pour the deglazing liquid over the scallops. Stir in 2 tbsp chopped fresh parsley and set scallops aside.

Cream Sauce

Start the roux

In the same skillet, reduce heat to medium and add 3 tbsp unsalted butter. When melted, sprinkle in 3 tbsp all-purpose flour and whisk continuously for 1–2 minutes to cook the raw flour taste, forming a blond roux. Do not let it brown deeply.

Add liquids

Gradually whisk in 1 cup whole milk and 1/2 cup heavy cream, followed by 1/2 cup low-sodium chicken or seafood stock. Whisk constantly to keep the sauce smooth. Bring to a gentle simmer; it will thicken in 2–4 minutes.

Season and finish

Stir in 1/2 tsp salt, 1/4 tsp pepper, 1/2 tsp Dijon mustard, 1/4 tsp Worcestershire sauce and 1/2 tsp fresh thyme leaves (if using). Continue to simmer 1–2 minutes to meld flavors. Remove from heat and whisk in 1/2 cup grated Parmesan until melted and smooth. Taste and adjust seasoning.

Combine scallops with sauce

Return the seared scallops and any collected juices to the skillet and gently fold them into the cream sauce to coat. Warm through for 30–60 seconds; do not simmer vigorously to avoid overcooking the scallops.

Topping & Assembly

Preheat oven and prepare baking dish

Preheat the oven to 375°F (190°C). Butter a 9x9-inch (or similar 2-quart) shallow baking dish lightly.

Assemble casserole

Pour the scallops and cream sauce into the prepared baking dish, spreading evenly.

Make topping

In a small bowl, combine 3/4 cup plain dry breadcrumbs, 1/2 cup grated sharp cheddar (or extra Parmesan), 1 tsp lemon zest, and 1 tbsp chopped fresh parsley. Stir in 2 tbsp melted butter until the mixture is evenly moistened and crumbly.

Top and bake

Sprinkle the breadcrumb-cheese mixture evenly over the scallop mixture. Bake in the preheated oven for 18–22 minutes until the topping is golden and the filling is bubbly at the edges.

Finish and serve

Remove casserole from oven and let rest 5 minutes. Garnish with remaining chopped parsley. Serve hot with lemon wedges on the side if desired.