Cook whole zucchini in boiling salted water until barely tender (7 to 10 minutes). Cut in half lengthwise; scoop squash from shells and mash.
Cook sausage with onion; drain fat. Stir in zucchini. Reserving 2 tablespoons Parmesan cheese, add remaining ingredients.
Spoon into shells and place in shallow baking dish. Sprinkle with cheese and dash with paprika. Bake at 350 degrees for 25 to 30 minutes. Makes 4 servings.