In a large, deep skillet, cook the bacon over moderately high heat until browned, about 8 minutes. Add the onion and garlic, reduce the heat to moderate and cook, stirring, until softened, about 5 minutes. Add the stock, season with salt and pepper and a pinch of sugar and cook until the liquid has reduced to 1 cup, about 12 minutes.
Meanwhile, in a large pot of boiling salted water, blanch the brussels sprouts until barely tender, about 3 minutes.
Add the sprouts to the skillet. Simmer gently over moderate heat, stirring occasionally, until tender throughout, about 10 minutes; season with salt and pepper. Using a slotted spoon, transfer to a bowl. Boil the liquid in the skillet over moderately high heat until reduced to 1/2 cup. Pour the sauce over the brussels sprouts and serve.