Pasta with Butternut Squash and Sage

• 1 medium onion, chopped
• 2 tablespoons olive oil
• 1-pound butternut squash, peeled and cut into 2-inch pieces
• 3/4 cup water
• 1 teaspoon chopped fresh sage
• 1 pound gemelli or penne rigate pasta
• 2 tablespoons chopped fresh flat-leaf parsley
• 1 cup freshly grated parmesan plus additional for sprinkling
• 2 tablespoons unsalted butter (optional)

Cook onion in oil in a large nonstick skillet over moderately high heat, stirring occasionally, until golden. Finely chop squash pieces in a food processor and add to onion with water and salt to taste. Simmer, covered, stirring occasionally, 15 minutes, or until squash is tender. Add sage and simmer 1 minute more. Cook pasta in a 6-quart pot of boiling salted water until al dente. Reserve 1 cup cooking liquid in a cup and drain pasta. Return pasta to pot and add squash mixture, parsley, 1 cup parmesan, butter, and plenty of freshly ground black pepper, stirring until butter is melted. Season with salt and add some of reserved pasta cooking liquid to moisten if necessary. Serve sprinkled with additional parmesan.