Preheat the oven to 450° and position racks in the upper and lower thirds.
In a medium skillet, heat 2 tablespoons of the olive oil. Add the onion, cover and cook over moderately high heat until softened, 5 minutes; reduce the heat and cook, stirring, until very soft and lightly caramelized, 15 minutes.
Transfer the onion to a bowl and let cool slightly. Add the brussels sprouts, cheese and pancetta to the bowl and season with salt and pepper. Stir in 1/8 cup of the olive oil.
Lightly brush a rimmed baking sheet with olive oil. On a lightly floured work surface, shape dough into a rectangle about 1/4 inch thick. Transfer the dough to the baking sheet and brush with the remaining olive oil.
Spread the mixture on the dough and bake for 25 minutes, until golden and crispy.
Carefully remove the flatbread from the pan onto a work surface. Cut into rectangles and serve with lemon wedges.