In a large bowl, whisk together egg, 1/4 cup water, 1 tsp salt, and 1/4 tsp pepper. Stir in half the onion and half the garlic. Add bread-crumbs, cheese, pork, turkey, and 1/2 tsp Italian seasoning. Mix gently. Form into 16 balls.
Heat 1 Tbs oil in a large non-stick skillet over med-high heat. Add the meatballs; brown on all sides, 4 to 6 minutes. Transfer to a plate with a slotted spoon. Cook remaining meatballs in remaining Tbs of oil; remove meatballs.
Add remaining onion; cook over medium-low until soft, about 5 minutes. Add remaining garlic and 1/2 tsp Italian seasoning; cook 30 seconds. Season with salt and pepper. Stir in tomatoes and 3/4 cup water. Return meatballs; cover and simmer until cooked through, about 20 minutes. Remove meatballs.
Meanwhile, in a large pot of boiling salted water, cook spaghetti according to package directions until al dente. Drain; return to pot. Toss with sauce; serve meatballs on top, sprinkled with more cheese.