NOTE: if you don’t have an oven safe skillet, you can sear them then transfer to a shallow roasting pan.
Preheat the oven to 375°. Thaw the frozen spinach then squeeze it dry using paper towels. It’s important the spinach is free from its moisture or the meatballs will be too wet and won’t hold together.
Add the ground chicken, egg, parmesan cheese, bread crumbs, Italian seasoning, salt, pepper, garlic powder and the thawed/dried spinach then blend using your hands or a spoon.
Form into approximately 1 1/2″ meatballs. I recommend a scooper if you have one so the meatballs are the same size and bake uniformly. If not, try to make them as close in size as possible.
Add the olive oil to an oven-safe skillet and once it’s hot, add the meatballs and sear them on two sides.
Add one cup of the marinara to the skillet, spreading it around the meatballs. Top each meatball with about one tablespoon of the remaining marinara, dividing it between them then sprinkle with shredded mozzarella.
Bake 15-18 minutes or until the meatballs reach an internal temp of 165°. Optional: broil 1-2 minutes to brown the mozzarella. Serve right away.