In large skillet, brown Italian sausage, onion and garlic over medium heat; drain. Stir in salt.
Cook spaghetti according to package directions in boiling salted water (2 quarts water, 2 teaspoons salt); drain and rinse.
Heat oven to 350 F. Lightly butter a 9×13 baking dish; set aside.
In small bowl, combine milk and egg; stir into spaghetti. Sprinkle with Parmesan, tossing to coat.
Pour mixture into baking dish, spreading evenly.
Spoon Italian sausage over spaghetti mixture. Spoon pasta sauce evenly over top; sprinkle with cheese.
Bake until heated through, around 25-30 minutes. Remove from oven and sprinkle with parsley. Let stand 5 minutes, covered; cut into squares to serve.