Divide the ground chuck into two 8oz portions. Gently form each portion into a patty about 3/4" thick, making a slight indentation in the center with your thumb to prevent doming. Avoid overworking the meat.

Season both sides of each patty with kosher salt and freshly ground black pepper just before cooking.

If using a grill or skillet, preheat to medium-high heat (about 400°F/200°C). Add 1 teaspoon oil to the pan if using a skillet. Cook patties 3-4 minutes per side for medium-rare to medium, or longer to desired doneness. During the last minute of cooking, place 2 oz (approx) grated/sliced gruyere on each patty and cover the pan or close the grill to melt the cheese.

Caramelized Onions

Heat a wide skillet over medium-low heat. Add 1 tbs olive oil and 2 tbs unsalted butter; when butter melts and starts to foam, add the thinly sliced onions in an even layer.

Season onions with 1/2 tsp kosher salt, 1/4 tsp black pepper, and 1/2 tsp granulated sugar to promote browning. Cook gently, stirring every 4-6 minutes, until deeply golden brown and soft, about 30-40 minutes total. If the pan browns too quickly, lower heat and add a splash of water to deglaze.

Optional: during the last 3 minutes, add 2 tbs dry white wine or sherry to deglaze the pan and scrape up browned bits; cook until liquid evaporates and onions are jammy. Keep warm until assembly.

French Onion Jus

In a small saucepan over medium heat, add 1/4 cup red wine and bring to a simmer to reduce by half (about 2-3 minutes).

Add 1 cup beef demi-glace (or concentrated beef stock) to the reduced wine, 1 tsp Worcestershire sauce and 1 thyme sprig. Simmer gently until sauce coats the back of a spoon, about 6-8 minutes. Taste and season with 1/4 tsp salt and 1/8 tsp pepper as needed.

Off heat, whisk in 1 tbs cold butter to finish the jus for shine and mouthfeel. Remove thyme sprig before serving and keep the jus warm.

Assembly & Finishing

Lightly butter the cut sides of the brioche buns (1 tbs total) and toast them in a hot skillet or on the grill until golden, about 1-2 minutes per side. Keep warm.

Place the melted-gruyere-topped burger patty on the bottom bun. Spoon a generous portion (about 2-3 tbs) of caramelized onions over the cheese.

Ladle 1-2 tbs of the warm French onion jus onto the patty over or around the onions so it soaks slightly into the burger. Sprinkle finely chopped chives (about 1 tsp) over the onions.

Cap with the toasted top bun. Serve immediately with extra jus on the side for dipping.