In a mixing bowl, combine the ground turkey, diced yellow onion, egg whites, grated Parmesan, whole-wheat panko breadcrumbs, and minced garlic; mix until combined and form into 1-inch meatballs.
Heat the olive oil in a large high-sided skillet over medium-high heat.
Add the meatballs and cook 8–10 minutes, turning once or twice, until browned and cooked through; drain any excess liquid from the skillet.
Return the skillet to medium-high heat and add the diced tomatoes, marinara sauce, hot water, and the whole-wheat spaghetti (break noodles in half if needed to fit).
Bring the mixture to a boil, then reduce heat to a simmer, cover, and cook 10–12 minutes until the pasta is tender, stirring occasionally and adding more water if necessary.
Serve hot and top with additional grated Parmesan or chopped parsley if desired.