Pasta with Butternut Squash and Rosemary

1 cup chopped onion

1 cup finely chopped celery

1 teaspoon minced fresh rosemary or 1/2 teaspoon dried

3 1/4 cups 1/3-inch pieces peeled butternut squash

8 ounces plum tomatoes, peeled, seeded, chopped

1/2 cup water

2 1/2 ounces 1/4-inch-thick prosciutto slices, finely chopped

2 tablespoons chopped fresh parsley

12 ounces conchiglie (medium pasta shells)

3/4 cup freshly grated Parmesan cheese