Preheat Oven & Prepare Baking Sheet: Preheat oven to 400°F (200°C). Line a baking sheet with parchment paper.
Mix Meatball Ingredients: In a large bowl, combine ground chicken, Parmesan, breadcrumbs, egg, parsley, garlic, Italian seasoning, salt, and pepper. Gently mix until just combined.
Form Meatballs: Roll the mixture into 1.5-inch meatballs and place them evenly on the prepared baking sheet.
Bake Meatballs: Bake for 20-25 minutes, until internal temperature reaches 165°F. Optionally, broil for 2-3 minutes for a golden-brown top.
Prepare Sauce: In a large skillet, melt butter over medium heat. Stir in flour and cook for 1 minute. Add garlic, Italian seasoning, red pepper flakes, and salt.
Add Liquids: Slowly whisk in half-and-half and chicken broth. Cook until thickened, about 4-5 minutes.
Incorporate Spinach: Stir in fresh spinach and cook until wilted.
Combine & Serve: Add the baked meatballs to the sauce, coating evenly. Serve warm, garnished with extra Parmesan.