6 tablespoons extra-virgin olive oil, plus more for brushing
1/2 cup mayonnaise
2 tablespoons fresh lemon juice
Salt and freshly ground pepper
8 slices of thick-cut bacon (12 ounces)
3 pounds mixed ground chuck and ground sirloin
2 Tbsp Worcestershire sauce
1 Tbsp Oregano
12 ounces brie, sliced
8 brioche burger buns, split
Preheat the oven to 400°. On a rimmed baking sheet, toss the onions with 2 tablespoons of the olive oil and roast until softened, about 20 minutes.
Meanwhile, in a small bowl, whisk the mayonnaise with the lemon juice. Gradually whisk in the remaining 1/4 cup of olive oil and season with salt and pepper.
In a large skillet, cook the bacon over moderately high heat until crisp. Drain on paper towels and break each strip in half.
Light a grill or preheat a grill pan. Mix the Ground meat with the Worcestershire sauce and Oregano. Form the ground meat into eight patties. Season generously with salt and pepper and brush with olive oil. Grill over moderately high heat until well-browned on the bottom, 5 - 7 minutes. Flip the burgers, mound the brie on top and grill for 4 - 5 minutes longer, until the cheese is slightly melted and the burgers are medium-rare. Cook longer on both sides for meat doneness that is desired.
Spread the lemon mayonnaise on the cut sides of the buns; Place on Grill for 2 min to toast; set the burgers on the bottoms and top with the bacon and onions. Close the burgers and serve right away.