In a large skillet, cook the bacon over medium-high heat until crispy, 6 to 8 minutes. Transfer to a plate to cool slightly. Pour off most of the bacon fat into a heat-safe bowl, leaving a thin layer of fat in the skillet.
Return the skillet to medium-high heat and cook the onions until translucent, 5 to 7 minutes; season with salt. Add the Brussels sprouts and cook, stirring occasionally, until browned and tender, another 7 to 10 minutes, adding more bacon fat if the pan seems dry. Coarsely chop the bacon and then stir it into the Brussels sprouts. Add salt and pepper to taste. Finish with the lemon juice and serve with hot sauce if desired.