Place brussels sprouts into slow cooker. Stir in olive oil and season with salt and pepper, garlic powder, onion powder and red pepper flakes. Top with butter.
Cover and cook on low heat for 3-4 hours or high for 1-2 hours.
To make the balsamic reduction, add balsamic vinegar and brown sugar to a small saucepan over medium heat. Simmer and reduce by half (it takes around 6-8 minutes), then set aside and let it cool.
Serve brussels drizzled with balsamic reduction and topped with Parmesan cheese.