Cook the Bacon: Place the bacon in a large skillet or on a griddle over medium heat. Cook until crisp, then transfer to a plate lined with paper towels.
4 slices bacon
Cook the Onions: Add the sliced onion to the skillet or griddle with a little of the bacon fat. Cook for 8 to 10 minutes, stirring often, until soft and browned. Move the onions to a plate.
1 medium onion
Season the Beef: In a medium bowl, gently mix the ground beef with the salt, black pepper, and garlic powder until just combined.
1 pound ground beef, 1 teaspoon salt, 1/2 teaspoon black pepper, 1 1/2 teaspoons garlic powder
Form the Meat Balls: Divide the beef mixture into 4 equal portions and shape them into loose balls.
Smash the Patties: Heat the skillet or griddle over medium-high to high heat. Place the beef balls on the hot surface and press each one down firmly with a spatula or burger press until thin.
Cook the First Side: Let the patties cook without moving them for 2 to 3 minutes, until the bottoms are deeply browned and crisp around the edges.
Flip and Top the Patties: Flip each patty and cook for 1 to 2 minutes more. Top each patty with some cooked onion, 1 slice of bacon, and 1 slice of cheddar cheese.
4 slices cheddar cheese
Melt the Cheese: Pour the water onto the hot griddle away from the burgers, then quickly cover with a lid or melting dome for about 30 to 60 seconds, until the cheese is melted.
1 tablespoon water
Toast the Buns: While the cheese melts, place the brioche buns cut-side down on the skillet or griddle and toast until lightly golden, about 1 minute.
2 brioche buns
Assemble the Burgers: Place 2 patties on the bottom half of each bun to make 2 double burgers. Add any optional toppings, then place the top bun on.
Ketchup
Serve: Serve immediately while hot and gloriously messy.