To make the meatballs, heat the oil in a large skillet over medium-high heat. Add the onion and garlic and cook for 3-4 minutes, until softened.
Transfer the onions and garlic to a mixing bowl and let cool slightly.
Stir in the egg, bread pieces, breadcrumbs, and seasonings. Let sit for 5 minutes.
Mix in the ground beef, spinach, and parmesan.
Using a meatballer or damp hands, form the mixture into 12-16 meatballs.
Return the skillet to the stove. Add the meatballs in a single layer and cook 304 minutes per side, until browned.
Add the sauce ingredients to your slow cooker. Stir well to combine. Gently add the meatballs to the sauce.
Cover and cook on low for 4-6 hours.