Remove casings from sausage, and discard. Cook sausage in a large skillet over medium-high heat, stirring until crumbled and no longer pink. Drain well, reserving 1 teaspoon sausage drippings in skillet.
Sauté chopped mirliton and next 4 ingredients in hot drippings in skillet 5 minutes or until onion is tender. Stir in sausage, spaghetti sauce, and tomato sauce; cover and cook over medium heat, stirring occasionally, 30 minutes or until mirliton is tender. Keep warm.
Bring 3 cups water and salt to a boil in a saucepan; gradually stir in polenta. Reduce heat, and cook, stirring constantly, 5 minutes. Stir in milk and half of Parmesan cheese; cook 3 minutes. Pour polenta onto a serving plate; spoon sausage mixture evenly over polenta, and sprinkle with remaining Parmesan cheese.
* 4 large yellow squash or zucchini may be substituted.