Preferably in an iron skillet (or another oven-safe pan) with about 1 tbls olive oil, sear the meatballs, turning often to brown all sides. They do not need to be cooked through.
Pour in the jar of marinara, and add spinach. Make sure spinach and meatballs are incorporated into the sauce.
Add dollops of ricotta around the pan.
Put the pan in the oven and bake for 15-20 mins until the internal temperature of the meatballs is at least 165.
Sprinkle shredded mozzarella on top of the meatball bake and return to oven, set to broil.
Remove when the cheese has browned and looks amazing!
Notes
Eat as is for Keto, or for the non-keto eaters, you can add to a hoagie for a meatball sub!