Preheat oven to 400 degrees.
Heat olive oil in a small skillet, add onions and garlic and cook over medium heat, stirring occasionally, until softened, about 10 minutes.
Remove from heat and let cool.
In a large mixing bowl, combine beef, turkey, carrot, spinach, bread crumbs, parmesan cheese, egg, salt and pepper (use a fork or your hands to mix until combined).
Stir in half of the cooked onions and garlic mixture (keeping the rest aside for the sauce).
Using your (well washed) hands, form meat mixture into 1 1/2" balls and space equally on nonstick baking sheets.
Bake in the centre of the oven for approximately 15 minutes or until cooked through (break open a meatball to check).
Use remaining cooked onion and garlic mixture and add to low sodium tomato sauce and season with salt and pepper.
Bring sauce to a boil, reduce heat and simmer while you cook the noodles in another pot.
In a large pot of boiling, salted water, cook spaghetti until al dente.
Drain, rinse and top with sauce and meatballs.
Serve immediately with freshly grated parmesan cheese.