Preheat oven to 375°F. In a large pot of boiling salted water, cook spaghetti, stirring often, until al dente, about 10 minutes. Drain well in a colander. In a large, wide saucepan or deep skillet, heat tomato sauce over low heat while you proceed.
While pasta is cooking, combine beef, soaked bread crumbs, cheese, onion, parsley, egg, garlic and salt in a large bowl. Form mixture into balls slightly larger than golf balls. (They will shrink as they cook.) Lay meatballs out on two nonstick or oiled baking sheets and bake in oven until nicely browned, about 10 minutes.
Transfer meatballs from oven with tongs and place into simmering sauce to combine. Serve meatballs and sauce on top of spaghetti.