In a large, deep skillet, cook the bacon over moderately-high heat until browned, about 8 minutes. Using a slotted spoon, transfer the bacon to paper towels to drain. Add the onion and garlic to the pan, reduce the heat to moderate and cook, stirring, until softened, 3 to 4 minutes. Remove the softened vegetables from the pan and set aside. Add the Brussels sprouts in batches and continue to cook, stirring occasionally, until they are golden brown on the outside. Add the reserved bacon and vegetables to the Brussels sprouts in the pan along with salt and pepper, to taste, and a pinch of sugar. Cover skillet with a lid and cook, stirring occasionally, until sprouts are just tender, 10 to 12 minutes.