Heat oven to 400ºF. Place a piece of parchment paper over baking sheet.
Bring a large pot of water to boil for pasta.
Place chicken and spinach in bowl. Grate 1 clove of garlic into bowl or finely chop and add garlic. Grate 3-4 tablespoons of onion directly into bowl. Combine 1/2 cup of cheese and breadcrumbs with nutmeg and barely moisten with milk, no more than 1/4 cup. Sprinkle the bread-cheese mixture into chicken then add egg, parsley, salt and pepper and 2 tablespoons tomato paste.
Combine 1 tablespoon of tomato paste with 1 tablespoon EVOO in small bowl. Mix and combine the chicken with breadcrumbs and section the meatball mix into quarters. Then roll 3 large balls from each section. Arrange balls on baking sheet as you work. Using a pastry brush, brush meatballs with tomato paste and oil mixture and roast for 18-20 minutes.
Meanwhile, heat 2 tablespoons EVOO in a large skillet over medium-low heat. Finely chop 2 cloves of garlic and the remainder of the onion. Add the garlic and onion to the EVOO and gently cook for 8-10 minutes. Add tomatoes and stock to the pan and gently mash up. Season with salt and pepper. Stir in basil and allow the sauce to reduce while you cook pasta. Cook pasta to al dente, and reserve 1 cup of starchy water. Then drain pasta and place back in hot pot with butter, remaining cheese, and sauce. Toss pasta to coat evenly, adjust salt and pepper and serve along side the chicken meatballs.