Mushroom Croque-monsieur Recipe

Prepare mushrooms

Clean mushrooms by wiping with a damp cloth and slice them into even pieces (about 1/4" thick for cremini; tear oyster/shiitake if large).

Heat a large skillet over medium-high heat. Add 1 tbs butter and 1 tsp olive oil. When the butter foams, add the sliced mushrooms in a single layer (work in batches if needed) and let them brown without stirring for 2 minutes.

Stir and continue cooking, allowing mushrooms to release liquid and then brown, about 4–6 more minutes. Add the minced shallot and cook 1–2 minutes until fragrant.

Add the minced garlic and cook 30 seconds. Pour in 2 tbs dry white wine (or vermouth) to deglaze, scraping any browned bits. Cook until most liquid evaporates, about 1–2 minutes.

Stir in 1 tbs chopped thyme and 1 tbs chopped parsley. Season with 1/2 tsp salt and 1/4 tsp black pepper. Remove from heat and let cool slightly — you should have about 2 cups cooked mushrooms.

Warm 1.5 cups of milk in a small saucepan or in the microwave until steaming but not boiling; keep warm.

In a medium saucepan over medium heat, melt 3 tbs butter. Whisk in 3 tbs flour and cook, stirring constantly, for 1–2 minutes to make a blond roux (do not brown).

Slowly pour the warm milk into the roux in a thin stream, whisking constantly to prevent lumps. Continue to cook and whisk until the sauce thickens and coats the back of a spoon, about 4–6 minutes.

Remove from heat and whisk in 1/2 tsp Dijon mustard, 1/4 tsp freshly grated nutmeg, 1/2 tsp salt and 1/8 tsp white pepper. Stir in 1/2 cup grated Gruyère until melted and smooth. Taste and adjust seasoning.

Preheat oven to 425°F (220°C). Line a baking sheet with parchment or lightly oil a shallow baking dish.

Spread about 1/2 tsp Dijon mustard on one side of 4 bread slices (these will be the bottom slices). Butter the outer sides of all 8 slices with the softened butter (about 4 tbs total) to ensure crisping in oven or pan.

Distribute the grated cheeses between the 4 bottom slices: sprinkle about 2 tbs Gruyère + 1 tbs Emmental on each. Top cheese with roughly 1/2 cup of the cooked mushroom mixture per sandwich (divide the 2 cups evenly). Season mushrooms lightly with freshly ground black pepper (about 1/8 tsp per sandwich).

Place the remaining 4 bread slices, buttered side out, on top to form sandwiches.

Heat a large nonstick or cast-iron skillet over medium heat. Working in batches if necessary, add the sandwiches butter-side-down and toast until golden, about 2–3 minutes per side, pressing gently with a spatula. Transfer toasted sandwiches to the prepared baking sheet, buttered sides up.

Spoon a generous 3–4 tbs (about 2–3 tbsp) of the warm Mornay sauce over the top of each sandwich so sauce covers the surface. Sprinkle each with additional grated cheese (about 2 tbs) to create a gratinée.

Place the baking sheet in the preheated oven and bake until the sauce is bubbling and the tops are golden brown, about 6–8 minutes. For extra browning, switch to broil for 1–2 minutes watching carefully to avoid burning.

Remove from oven and let rest 2 minutes. If using, brush the cut sides with the beaten egg before final broil step earlier (optional for extra sheen). Sprinkle with flaky sea salt and chopped parsley. Slice sandwiches in half and serve immediately.