1 tbsp. olive oil, plus more for drizzling
3 tbsp. butter, at room temperature
2 large or 3 medium onions, quartered and thinly sliced
Salt and finely ground black pepper
½ cup beef consommé or beef stock
1 cup sour cream
8 slices meaty bacon
1 1/2–2 lbs. 80% lean ground beef
Kosher salt and coarsely ground black pepper
2 tbsp. Worcestershire sauce
4 slices Swiss cheese
4 slices white American cheese
4 brioche or potato burger rolls, split
2 tbsp. Dijon mustard
Pickle relish or sliced sandwich-size pickles
Leaf lettuce or little gem leaves
Preheat the oven to 375°. In a large skillet, heat 1 tbsp. oil, one turn of the pan, over medium. Add 1 tbsp. butter; when it melts, add the onions, season with salt and finely ground pepper and cook, stirring occasionally, until deep golden brown, 30 to 35 minutes. Transfer to a medium bowl and mix in the consommé; let cool. Mix in the sour cream.
Meanwhile, arrange the bacon in a single layer on a slotted broiler pan or on a wire rack set inside a rimmed baking sheet. Bake until very crispy, about 30 minutes. Cut the bacon in half crosswise. Preheat the broiler.
In another medium bowl, season the beef with kosher salt and coarsely ground black pepper; mix in the Worcestershire sauce. Form into 4 patties (thinner in the centers for even cooking).
Heat a cast-iron skillet or griddle over medium-high. Drizzle the skillet with oil, add the patties and cook 7 to 8 minutes, turning occasionally. Top each patty with 1 slice each of Swiss and American cheese during the last minute or two of cooking; tent loosely with foil to melt the cheese.
Toast the buns under the broiler until golden brown, about 2 minutes.
In a small bowl, mix the mustard and remaining 2 tbsp. butter; spread over the bun bottoms. Top each with relish or pickles, lettuce, a cheeseburger and 4 half-slices of crispy bacon so every bite gets an even amount of bacon. Slather the bun tops with onion sauce and set in place.