Preheat your grill to medium high heat and season your burgers with salt and pepper.
Grill onion over direct heat, flipping occasionally until onions have become slightly translucent and taken on dark grill marks, remove from the heat. (For a stovetop method, melt 1 tablespoon butter in a pan, add onions and 1 teaspoon salt. Let cook over medium heat, stirring often for 20 minutes.)
Grill the burgers 4-5 minutes, flip. Sear burgers 3-4 more minutes or until the internal temperature reaches 160°F.
Build your burgers with fig spread on the bottom of the bun, followed by the burger, brie, grilled onions, mixed greens, and finish with toasted bun top.