Heat a skillet over medium heat. Cook ground turkey, Italian sausage, garlic, salt, egg, black pepper, shallot, and breadcrumbs until just combined to avoid overmixing.
Shape the mixture into golf ball-sized meatballs. Cook the meatballs in the skillet, turning them gently, until browned on all sides and cooked through, about 10-12 minutes. Set aside.
Soup
Heat olive oil over medium heat in a large pot. Add onion, carrots, celery, and garlic, sautéing until the vegetables are softened, about 5-7 minutes.
Stir in salt, pepper, and red pepper flakes. Add pesto and pasta to the pot, stirring to coat the pasta well.
Pour in chicken broth and bring to a boil. Reduce heat and simmer until pasta is tender, approximately 10-12 minutes.
A few minutes before serving, add spinach to the pot and cook until wilted, about 2 minutes.
Add the cooked meatballs to the soup. Stir gently to combine and heat through. Taste and adjust seasonings if necessary, then serve hot.
Notes
Grate Parmesan is optional for serving this recipe, but it adds another layer of flavor.