Caramelize Onions: Heat olive oil in a skillet over medium heat. Add onions, salt, and sugar. Cook, stirring occasionally, for 30–40 minutes until golden brown and soft.
Prep Puff Pastry: Roll out thawed puff pastry sheets. Cut into 48 circles (~2.5 inches) using a cookie cutter or cup. Make a smaller indentation in the center of each circle with a smaller cutter or knife tip.
Assemble Puffs: Place a piece of brie in each indentation. Top with caramelized onions. Brush edges with egg wash.
Bake: Preheat oven to 400°F (200°C). Bake for 12–15 minutes until puffed and golden.
Optional Finish: Drizzle with honey or add a dollop of fig jam before serving.