Place bacon in a large skillet and cook over medium-high heat, turning occasionally, until evenly browned, about 10 minutes. Drain the bacon slices on paper towels, retaining grease in the skillet. Crumble bacon.

Cook and stir onion in bacon grease over medium heat until translucent, about 5 minutes. Add Brussels sprouts; cook and stir until Brussels sprouts are lightly browned, about 5 minutes. Season with salt and pepper.

Sprinkle bacon over Brussels sprouts mixture and stir. Remove skillet from heat, transfer Brussels sprouts mixture to a serving bowl, and sprinkle with Parmesan cheese.