Preheat oven to 400 degrees. Line a baking sheet with parchment paper or aluminum foil for easy cleanup.
In a large bowl, add Brussels sprouts, onion, and bacon.. Drizzle with olive oil and sprinkle with salt, pepper, and red pepper flakes (if using). Toss to coat and spread into a single layer on prepared baking sheet.
Bake until Brussels sprouts are tender-crisp and browning around the edges, 30 to 35 minutes, stirring halfway through baking time. Drizzle with vinegar and toss to coat. Season to taste with salt and pepper.
Brussels sprouts: Seek out sprouts of a similar size, if possible. When everything is cut in the same general size, it's easier to ensure even cooking. Slicing the sprouts in half increases flat surface area, too; flat surfaces make more contact with the baking sheet, which helps boost the caramelization factor.
Red onion: Chopped shallots would also work wonderfully.
Crushed red pepper flakes: These are completely optional. Add a little (or a lot) if you like your sides on the spicy side.
Yield: This recipe makes eight generous 1-cup servings of Roasted Brussels Sprouts with Bacon.
Storage: Store leftovers covered in the refrigerator for up to 4 days.