1 stick butter, divided
2 large onions, thinly sliced plus 1/2 small onion
1 tablespoon fresh thyme leaves or 1/4 teaspoon dried
1 bay leaf
Salt and pepper
1/2 cup beef stock
1 1/2 pounds 90% lean ground beef of the best quality, at room temperature
1 tablespoon Worcestershire sauce
2 cloves garlic, grated or pasted
2 tablespoons olive oil
4 slices sharp cheddar
4 slices Gruyère or Swiss cheese
1 bunch upland cress
8 slices (each about 3/4-inch thick) white bread
In a large skillet, heat 4 tablespoons butter over medium. When it foams, add the sliced onions, thyme and bay leaf; season with salt and pepper. Cook, stirring frequently, until onions are caramel in color and very soft, 20-30 minutes. Add the stock during the last 5 minutes of cooking. Remove from heat and reserve.
Heat a cast-iron skillet over medium-high heat.
Place beef in a medium mixing bowl. Using a hand grater or box grater, grate 1/2 small onion over the meat. Add Worcestershire, garlic, salt and pepper. Using your hands, combine and form into 4 patties (thinner in the centers for even cooking).
Add olive oil to the skillet. Add the burgers and cook 4 minutes on each side for medium (longer for more well-done).
Top each patty with 1 slice of each cheese during the last minute or two of cooking, tenting with foil to melt.
Toast bread and butter one side of each slice of toast. Divide the upland cress among 4 slices of toast and top cress with the buttery burgers some buttery onions and the remaining 4 slices toast.