Begin by thinly slicing the sweet onions. In a skillet, heat a tablespoon of olive oil over medium-low heat and add the onions, seasoning them lightly with kosher salt and freshly ground black pepper. Stir frequently, cooking for about 30-45 minutes until they are wonderfully soft and golden brown.

While the onions are caramelizing, mix together the baconnaise sauce by combining mayonnaise, finely chopped cooked bacon, Worcestershire sauce, and garlic powder in a bowl. Stir until thoroughly blended.

Divide the ground chuck into eight equal balls, roughly the size of a golf ball. Preheat your griddle or cast-iron skillet over medium-high heat until it's sizzling hot.

Place the balls onto the hot surface, using a spatula to smash each ball down firmly, and sprinkle with salt and pepper. Cook for about 3-4 minutes until the edges look crispy.

Carefully flip the patties, and immediately add a slice of American cheese on top of each patty, then add the caramelized onions. Cook for another 1-2 minutes until the cheese is melted and bubbly.

Spread a generous layer of the smoky baconnaise sauce on the bottom half of each toasted brioche bun. Layer with a piece of lettuce, followed by the cheesy, juicy burger patties, and top with fresh slices of tomato. Place the top bun to complete the burger.