Heat 1 tbsp olive oil in a skillet over medium-low heat. Cook the thinly sliced yellow onion slowly for 20-30 minutes, stirring occasionally, until deeply golden brown and sweet, then set aside.

Gently combine ground beef, minced garlic, salt, black pepper, and onion powder in a bowl, being careful not to overmix. Divide the seasoned meat into 8 equal, loosely packed balls, about 2-3 ounces each, but do not press them into patties yet.

Place a large cast-iron skillet or griddle over high heat until very hot and just smoking. Add 1 tablespoon of butter or high-smoke-point oil to coat the surface evenly.

Place 2-3 beef balls on the hot surface with space between them. Immediately cover each ball with parchment paper and use a sturdy spatula to press firmly and evenly, smashing it into a thin, ¼-inch thick patty for crispy edges.

Cook for 2-3 minutes until the edges are crispy and browned, and you see delicious browning on the bottom.

Carefully flip each patty with your spatula and immediately place a slice of American cheese on top.

Continue cooking for another 1-2 minutes until the cheese is melted and gooey and the second side is cooked through. While the burgers cook, quickly butter the cut sides of the brioche buns and toast them on the griddle for 30 seconds to 1 minute until golden brown.

Stack two cheesy patties on a toasted bun, and top with caramelized onions (if using) and any other desired toppings such as lettuce, tomato, or pickles. Serve immediately.