Preheat oven to 350 degrees F. Grease a 9x5-inch loaf pan.
Add 1/4 cup water to a large sauté pan with a lid. Bring to a boil over high heat. Reduce heat to medium. Add spinach and cover. Cook 1-2 minutes until the spinach wilts.
Remove from pan and squeeze out all moisture. Pat dry with a paper towel. Place spinach in a medium bowl. Empty pan and wipe dry.
Heat oil in sauté pan over medium heat. Add onion and sauté 5-10 minutes until translucent, but not brown. Stir occasionally. Add garlic and sauté 1 minute. Remove from heat and let cool. Add onion mixture to spinach. Add Mozzarella cheese and 2 Tablespoons Parmesan cheese. Season to taste with salt and pepper. Mix with a fork until uniform.
In a large bowl combine the ground meat, bread crumbs, egg and 3/4 of the jar of Mom’s Meatloaf Starter. Mix well with your hands or a wooden spoon until uniform. Place half of the mixture in the prepared loaf pan. Layer the spinach over the meatloaf mixture. Place the remaining meatloaf mixture over the spinach layer. Spread the remaining Mom’s Meatloaf Starter over the top and sprinkle with remaining Parmesan cheese.
Bake for 45-55 minutes or until a thermometer inserted into the center of the meatloaf registers 165 degrees F. Remove from oven and allow to rest 10 minutes. Cut into slices and serve.