Heat olive oil in a large skillet over medium heat.
Add chopped onion and cook until soft, about 3 minutes.
Add minced garlic and cook for 1 minute until fragrant.
Add ground beef, breaking it up with a spoon, and cook until browned.
Season with smoked paprika, salt, pepper, and red pepper flakes if using.
Push the beef to one side of the skillet and add Brussels sprouts cut-side down.
Cook without stirring for 5-7 minutes until Brussels sprouts are browned and crispy.
Stir everything together and cook for another 2-3 minutes until heated through.
Serve hot and enjoy!