If there's one thing that makes a pan of roasted Brussels sprouts even better, it's bacon. Brussels sprouts pair well with its smoky flavor, and the fat rendered during roasting helps the leaves grow lacy and crisp.
Place bacon on rimmed baking sheet and spread into an even layer. Place the baking sheet in the oven and heat to 400°F.
Place the Brussels sprouts in a large bowl or pot (something large enough to easily toss them), drizzle with the olive oil, sprinkle with the salt and pepper, and toss to combine. Remove the hot baking sheets from the oven. Stir the bacon. Arrange Brussels sprouts cut-side down into an even layer.
Roast for 15 minutes. Remove the baking sheet from the oven and stir the Brussels sprouts and bacon. Return the baking sheets to the oven, rotating them from front to back. Roast until browned and tender, 12 to 15 minutes more, depending on the size of the Brussels sprouts. Meanwhile, make the glaze.
Heat the apple cider, brown sugar, and apple cider vinegar in a small saucepan over high heat, stirring until the sugar is dissolved. Cook until it's reduced by about half and thickened, 10 to 15 minutes. Remove from the heat.
When the Brussels sprouts are ready, remove the baking sheet from the oven. Turn the oven off. Combine the Brussels sprouts onto one baking sheet, drizzle the glaze over the sprouts, and stir to combine. Return the baking sheet to the turned-off oven and let sit until the Brussels sprouts absorb some of the glaze, about 2 minutes more.