Start by melting 2 tablespoons of unsalted butter in a large skillet over medium heat. Once the butter is melted and bubbly, add 1 thinly sliced white onion and 1 finely chopped green bell pepper. Sauté the vegetables for about 6-7 minutes until they soften but do not brown.

Next, add 1 pound of lean ground beef (90/10 blend) to the skillet with the sautéed onions and bell peppers. Cook for about 5-6 minutes until the meat is browned and no longer pink.

Stir in ½ cup of beef stock, ¼ cup of ketchup, 2 tablespoons of Worcestershire sauce, and 1 teaspoon each of garlic powder, kosher salt, and black pepper. Simmer for about 4-5 minutes until the sauce begins to thicken.

Chop 8 ounces of provolone cheese into ½-inch pieces. Remove the skillet from the heat and stir the chopped cheese into the filling until it melts.

Preheat your oven to broil, slice brioche buns in half, and place them cut side up on a baking sheet. Toast for about 2-3 minutes, layer with provolone cheese, and top with the meat mixture.