Preheat oven to 350 degrees. Spray a 9 x 5-inch loaf pan with cooking spray.
Heat olive oil in a sauté pan over medium-high heat. Add onion and reduce heat to medium-low. Cook 4-5 minutes or until beginning to soften. Transfer 2/3 of the onion to a bowl to cool.
Mix in half the garlic.
Add spinach to sauté pan and cook briefly for any remaining moisture to evaporate.
Let cool slightly then stir in sun-dried tomatoes, feta cheese and salt and black pepper to taste. Add egg white and set aside.
To bowl with onions, add whole egg, breadcrumbs, milk, 2 tablespoons tomato sauce, basil, oregano, parsley, 1 teaspoon salt, black pepper and remaining 2 cloves minced garlic and mix well.
Mix in ground turkey or chicken.
Place 2/3 of the meat mixture in the loaf pan, creating a small well down the center.
Mound spinach mixture in the well. Top with remaining ground turkey.
Bake for 50-55 minutes or until a thermometer inserted in the center registers 165 degrees. Top with the remaining tomato sauce after 30 minutes, then bake to completion.
Let rest 5-8 minutes before slicing.