How to Make It
In a bowl, combine the yeast with the water and sugar and let stand until foamy, 5 minutes. Add the 1 tablespoon of oil, salt and 2 cups of the flour and stir until a soft dough forms. Knead in the remaining 1/2 cup of flour. Turn the dough out onto a lightly floured work surface and knead until firm yet supple, 5 minutes. Lightly brush the bowl with oil. Add the dough and cover with plastic wrap; let stand until doubled in bulk, 1 hour.
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 1/4 cup of the olive oil.
Preheat the oven to 450° and position racks in the upper and lower thirds. Lightly brush 2 rimmed baking sheets with olive oil. On a lightly floured work surface, cut the dough in half and shape each piece into a 13-by-15-inch rectangle 1/4 inch thick. Transfer the dough to the baking sheets and brush with the remaining 2 tablespoons of olive oil. Spread the topping on the flatbreads and bake for 25 minutes, until golden and crispy. Shift the pans from top to bottom and front to back halfway through. Slide the flatbreads onto a work surface. Cut into rectangles and serve with lemon wedges and Asian chile oil.