Preheat oven to 425 F. Line out an aluminum foil sheet pan.
Spread sprouts, onions and garlic on the pan over the brussels. Sprinkle with salt , pepper and red pepper flakes on the sprouts, then chop with the olive oil. Throw away the sprouts with clean hands until all is well coated.
Wiggle the pan into a single layer until the Brussels sprouts. Sprinkle the pieces of cut bacon evenly over the pan. Bake for 20 to 25 minutes, until fork-tender sprouts. If necessary taste and season with extra salt.