Place the ground beef in a bowl. Add Worcestershire, 1 tsp kosher salt and 1/2 tsp black pepper. Mix gently with your hands until just combined; avoid overworking. Divide into two equal portions (about 8 oz each) and gently form into 3/4–1 inch thick patties, making a slight indentation in the center of each with your thumb (helps prevent doming).

Heat a heavy skillet or cast iron pan over medium-high heat until very hot. Add 1 tbsp butter and 1 tsp vegetable oil to the pan. When butter foams, add the patties. Cook without moving 3–4 minutes until a deep brown crust forms. Flip and top each patty immediately with 2 slices of Swiss cheese (place slices slightly overlapping). Cook another 3–4 minutes for medium (internal temp ~140–145°F/60–63°C). Remove to a plate and tent with foil to rest while finishing gravy and assembly. Season remaining exposed sides lightly with salt if desired.

Mushroom duxelles

Begin duxelles

While the burgers cook, make the duxelles. Heat 1 tbsp butter and 1 tsp olive oil in a skillet over medium heat. Add the finely minced shallot and cook 1–2 minutes until translucent.

Cook mushrooms

Add the chopped mushrooms and a pinch of salt. Cook, stirring occasionally, letting moisture evaporate and mushrooms reduce. This will take 6–8 minutes. When most moisture is gone and mushrooms begin to brown, add the minced garlic and cook 30–45 seconds until fragrant.

Finish duxelles

Deglaze the pan with 2 tbsp white wine or sherry, scraping up brown bits. Add chopped thyme and continue to cook 1–2 minutes until liquid is mostly evaporated and mixture is fairly dry but glossy. Taste and adjust with up to 1/2 tsp additional salt and 1/4 tsp black pepper if needed. Keep warm until assembly.

Brown gravy

Make roux

In a small saucepan over medium heat, melt 2 tbsp butter. Whisk in 2 tbsp flour and cook, stirring constantly, 1–2 minutes until the roux is bubbly and just begins to brown lightly (deeper color = deeper flavor).

Finish gravy

Slowly whisk in 1.5 cups beef stock, breaking up any lumps. Add 1 tsp Worcestershire and 1 tsp soy sauce (if using). Bring to a gentle simmer and cook 3–5 minutes until slightly thickened to a gravy consistency. Season with 1/2 tsp black pepper and taste; add salt if needed (depending on stock sodium). Keep warm. If the gravy thickens too much, whisk in up to 2 tbsp water or more stock to reach desired consistency.

Assembly & finishing

Toast the brioche buns: spread softened butter on cut sides and toast in a skillet or under broiler until golden, about 1–2 minutes.

Build the burger

If you tented the patties, return them to the pan briefly over low heat (30–45 seconds) just to melt cheese fully and rewarm. Place one patty on the bottom bun. Spoon about 2–3 tablespoons mushroom duxelles over the cheese-covered patty. Place the second patty if making a double; for this recipe use one 8 oz patty per sandwich.

Spoon 2–3 tablespoons of warm brown gravy over the mushrooms (or serve extra on the side). Spread mayonnaise on the top bun if desired, then cap the burger. Serve immediately with remaining gravy on the side for dipping.