In a mixing bowl, combine the ground turkey, diced onion, egg whites, grated Parmesan, panko breadcrumbs, and minced garlic until evenly mixed; shape into 1-inch meatballs.
Heat the olive oil in a large high-sided skillet over medium-high heat.
Add the meatballs to the hot skillet and cook 8–10 minutes, turning once or twice so they brown evenly; drain any excess liquid from the pan.
Pour in the diced tomatoes (with juices), marinara sauce, and hot water; stir gently to combine and nestle the uncooked spaghetti into the sauce, breaking it if needed so it fits.
Bring the mixture to a boil, then reduce heat to a simmer, cover, and cook 10–12 minutes until the pasta is tender, stirring once halfway through and adding a splash more water if necessary.
Remove from heat and let sit a minute before serving; top with additional grated Parmesan or chopped parsley if desired.
