In a mixing bowl, combine the ground turkey, egg, Parmesan cheese, Italian seasoning, garlic powder, salt, and black pepper. Mix until evenly combined, then form the mixture into 1-inch meatballs.
Place the diced onion, sliced zucchini, chicken broth, diced tomatoes with their juice, and minced garlic into the slow cooker. Stir gently to combine.
Add the meatballs to the slow cooker, arranging them in a single layer if possible.
Cover and cook on low for 6 hours or on high for 3 hours until the meatballs are cooked through.
Stir in the fresh spinach during the last 20 minutes of cooking so it wilts but keeps its color.
Ladle the soup into bowls and top each serving with chopped fresh basil.