Arrange a rack in the bottom of the oven and heat the oven to 450°F. Meanwhile, remove any discolored outer leaves from Brussels sprouts and trim the ends. Keep small ones whole, and halve the remaining through the stem.

Place the Brussels on a rimmed baking sheet. Drizzle with olive oil, and season with salt, red pepper flakes, and some black pepper. Toss to combine, then arrange in a single layer cut-side down.

Roast until the sprouts are tender and lightly browned, 15 to 20 minutes. Flip the sprouts and roast until crispy and browned to your liking, 10 to 15 minutes more.

Place bacon in a single layer in a cast iron or other heavy-weight skillet. Cook over medium-high heat until the fat starts to render and the edges curl, about 2 minutes. Flip the bacon, reduce the heat to medium low, and continue to cook until crisp and golden-brown, 6 to 8 minutes more.

Transfer the Brussels to a serving bowl, drizzle with lemon juice, crumble in the bacon, and toss to combine. Taste and season with salt and pepper as needed, then sprinkle with the cheese, if desired.