1. 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.
2. Meanwhile, in a small bowl, whisk the mayonnaise with the
lemon juice. Gradually whisk in the remaining . cup of olive oil
and season with salt and pepper.
3. In a large skillet, cook the bacon over moderately high heat
until crisp. Drain on paper towels and break each strip in half.
4. Light a grill or preheat a grill pan. 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 minutes. Flip the burgers, mound the Brie on top
and grill for 4 minutes longer, until the cheese is slightly melted
and the burgers are medium-rare.
5. Spread the lemon mayonnaise on the cut sides of the buns;
set the burgers on the bottoms and top with the bacon and
onions. Close the burgers and serve right away.