Pre-heat oven to 375 degrees. Put all meatball ingredients in a medium sized bowl and mix together until everything is incorporated. Form 12 two inch balls. Push one cube of cheddar cheese into middle of each meatball. Heat up a cast iron skillet or oven safe pan on medium high heat and add 1 tbsp avocado oil or beef tallow. Then place meatballs in the pan. Do not crowd the pan, cook them in batches if necessary. Let the meatballs sear for about a minute on all sides until brown. Remove meatballs from pan and turn down to medium low heat. Sautee mushrooms until soft then add spinach and wilt. Add tomato sauce and cook on low for 5 minutes, stirring frequently. Pour resting juices from meatballs into pan and stir. Return meatballs to pan. Top with mozzarella cheese. Cover pan with tin foil and place in oven for 20 minutes. Remove foil, turn oven to broil and cook for another 3-5 minutes.