1. Fill a pot for the pasta with about 6 quarts of water, place over high heat, and bring to a boil.

2. Peel and finely chop the onion. Put the butter in a 12-inch skillet, add the chopped onion, and place over medium-high heat. Sauté until the onion just begins to turn a rich, golden color, about 5 minutes.

3. While the onion is sautéing, remove the casing from the sausage and break it up into pieces. When the onion is ready, add the sausage to the pan. Break the sausage up into small pieces with a wooden spoon and cook, stirring often, until the sausage is lightly browned, about 2 to 3 minutes.

4. While the sausage is cooking, wash the zucchini. Remove the ends, cut into quarters lengthwise, and then cut across into ½-inch chunks. When the sausage has lightly browned, add the zucchini. Season lightly with salt and cook until the zucchini begins to brown and is mostly tender, 6 to 8 minutes.

5. While the zucchini is cooking, peel the tomatoes and coarsely chop them. When the zucchini is ready, add the tomatoes, season lightly with salt, and cook until most of the liquid the tomatoes release has evaporated, 8 to 10 minutes.

6. When the tomatoes are halfway done, add about 2 tablespoons salt to the boiling pasta water, add the fusilli, and stir well. Cook until al dente.

7. When the pasta is done, drain well, toss with the sauce and the grated cheese, and serve at once.