Add olive oil to a skillet over medium heat. Once hot, add onion to the pan and cook until slightly softened, about 4 minutes. Add balsamic vinegar, garlic, herbs, salt and pepper and cook an additional 2 minutes. Turn off heat and let cool for a couple of minutes.
Meanwhile, add the remaining ingredients to a large mixing bowl. Add the cooled onion mixture and using clean hands mix together until thoroughly combined.
Using a medium cookie scoop, roll the mixture into balls and place on the baking sheet.
Bake for 22-25 minutes until the meatballs are cooked through and golden brown.
Use as desired or freeze for use at a later date.
Notes
*If using a flax egg - mix 1 tablespoon ground flaxseed with 2 1/2 tablespoons warm water. Let sit for 5-10 minutes until gelled. **Sub breadcrumbs or gluten-free breadcrumbs if desired for oat flour. For paleo - sub almond flour.