For the onions:
2 tablespoons butter
2 onions, chopped
Salt and fine black or white pepper
1 bay leaf
For the mushrooms:
3 tablespoons extra-virgin olive oil (EVOO)
1 pound cremini mushrooms, sliced
2 tablespoons fresh thyme, chopped
4 large cloves garlic, chopped
Salt and pepper
¼ cup sherry or ½ cup white wine
Juice of 1/2 lemon
For the burgers:
8 slices meaty bacon
1½ pounds (80/20) ground beef
3 tablespoons grated onion
Salt and pepper
2 tablespoons fresh rosemary, finely chopped
2 cloves garlic, grated or pasted
2 tablespoons Balsamic drizzle or aged Balsamic vinegar
2 tablespoons extra-virgin olive oil (EVOO)
8 slices fontina or mild provolone cheese
4 burger buns, split
4 slices beefsteak tomato
For the horseradish sauce:
½ cup sour cream
¼ cup aquafaba-based/chickpea mayo or mayonnaise
2 tablespoons horseradish
1 teaspoon black pepper
Juice of ½ lemon
¼ cup finely chopped chives or scallions
Preheat oven to 375˚F.
For the onions, in a skillet over medium heat, melt butter, add onions and season with salt and pepper, add a bay leaf. Cook onions, low and slow, to light caramel color, 20-25 minutes. Add 1 cup water, and cook over low heat until liquid is completely absorbed, 10 minutes more.
Meanwhile, for the mushrooms, preheat a second skillet over medium-high heat, add EVOO, 3 turns of the pan, and add mushrooms to brown, 10-12 minutes. Add thyme, garlic, and salt and pepper, and cook 5 minutes more. Add sherry or wine, let it absorb completely, turn off the heat, then add lemon juice.
Arrange bacon on slotted pan or wire-rack lined baking sheet and bake until crisp, 12-15 minutes. Mix together ingredients for horseradish sauce.
For the burgers, preheat a large cast-iron griddle or skillet over medium-high heat. Place beef in bowl. Using a handheld grater, grate onion over bowl so meat collects juices. Season with salt, pepper and rosemary. Add garlic, Balsamic and EVOO, mix into meat, and form 4 patties thinner at center and thicker at edges for even cooking. Cook patties 8 minutes, turning occasionally, melt 2 slices cheese on each patty.
Assemble burgers: bun bottom, horseradish sauce, bacon, sliced tomato, burger patty, mushrooms, onions, bun top.