Preheat oven to 425°F (220°C). Clean oyster shells under cold water. Shuck oysters, reserving liquor in a bowl, and place each oyster on half shell on a baking tray or in an ovenproof pan rimmed with rock salt or coarse crumbs to keep them level. If using already-shucked oysters, pat shells dry and arrange oysters on shells.

If large oysters, place 12 oysters (one per shell) so they sit flat and won't tip during baking.

Sauce Bienville (base)

Cook aromatics and mushrooms

In a medium skillet over medium heat, melt 3 tbs butter. Add the minced shallot and cook 1–2 minutes until translucent. Add the garlic and chopped mushrooms and cook 3–4 minutes until the mushrooms soften and any released liquid mostly evaporates.

Deglaze and reduce

Add 1 tbs dry white wine and 1 tbs stock to the pan to deglaze, scraping any browned bits, and let reduce by about half (about 1–2 minutes).

Make cream sauce

Reduce heat to medium-low and pour in 1 cup heavy cream. Stir in Dijon mustard, Worcestershire sauce, cayenne, salt, and black pepper. Simmer gently, stirring often, until sauce thickens enough to coat a spoon, about 4–6 minutes. Taste and adjust seasoning (add more cayenne for heat or salt if needed).

Seafood & Fillings

Add seafood and herbs

Stir the chopped shrimp and crab meat into the warm cream sauce, then fold in 2 tbs grated Parmesan, 2 tbs green onion, and 1 tbs parsley. If sauce is very loose, simmer 1–2 minutes more to tighten. Remove from heat and, if desired, stir in up to 2 tbs reserved oyster liquor for briny flavor (do not overdo to avoid thinning).

Spoon about 1–2 tbs of the Bienville mixture over each oyster, covering the oyster meat but not overflowing the shell.

Topping/Breadcrumbs

Finish and bake

Combine 1 tbs breadcrumbs with 1 tbs grated Gruyère or extra Parmesan and drizzle with 1 tsp melted butter, tossing to coat. Sprinkle a light layer of the breadcrumb-cheese mixture over each sauced oyster.

Place the tray in the preheated oven and bake until the topping is golden and sauce is bubbling, 8–10 minutes. For a browned crust, switch to broil for 1–2 minutes watching carefully so it doesn’t burn.

Remove oysters from oven and let rest 1–2 minutes. Garnish with a small sprinkle of chopped parsley and serve immediately with lemon wedges.