4 tablespoons butter
2 large onions, chopped
Salt and white pepper
1 teaspoon sugar
½ cup beef bone broth or consomme
2 tablespoons olive oil
1 pound cremini mushrooms, wiped clean and sliced
2 tablespoons fresh thyme leaves, chopped, or 2 teaspoons dried
4 cloves garlic, chopped, or 2 teaspoons dried
½ cup white wine or ¼ cup dry sherry
1 ½ pounds ground sirloin, 85%
2 tablespoons Worcestershire sauce
Non-aerosol olive oil cooking spray
¾ cup steak sauce, such as A1
8 slices (cut on the thick side) Swiss cheese
4 brioche or other burger rolls of choice
Heat a medium skillet over medium heat. Add 2 tablespoons butter and melt. Add onions and season with salt and white pepper. Add the sugar and cook, stirring frequently, until very soft and light caramel in color, 20 to 25 minutes. Add bone broth or consomme and reduce heat to warm/low.
Meanwhile, heat olive oil, 2 turns of the pan, in a medium nonstick skillet over medium-high heat. Melt 2 tablespoons butter into the oil, then add mushrooms and brown. Season with salt and pepper, add thyme and garlic and stir a minute, then add wine or sherry and reduce heat to warm/low.
Meanwhile, heat a cast-iron skillet over medium-high heat. Place meat in bowl and season with Worcestershire, salt and pepper, then form 4 patties thinner at the center for even cooking. Spray the pan with oil and cook the patties for 4 minutes, then flip, baste liberally with steak sauce and cook 4 minutes more. Top the burgers with 2 slices cheese each and build burgers: bun bottom, Dijon, mushrooms, patties, onions, bun top.