Heat a small skillet over medium heat and spray with cooking spray. Cook and stir onion and garlic in the skillet until translucent, about 5 minutes.
Heat a griddle over medium heat and spray with cooking spray.
Combine oats, sweet potatoes, egg, sage, and black pepper together in a bowl; add onion mixture. Shape mixture into golf ball-size balls and slightly flatten.
Cook sweet potato balls on the preheated griddle until browned, about 4 minutes per side.
