1. Preheat oven to 400ºF. Line a baking sheet with heavy-duty aluminum foil. On baking sheet, toss squash with oil and 1/4 tsp. salt to coat. Spread squash in a single layer and roast until soft, stirring occasionally, about 25 minutes.
2. Bring a large pot of salted water to boil. Cook spaghetti until just tender, about 10 minutes, or as package label directs. Drain, reserving 1/2 cup of pasta cooking water. Return spaghetti to pot and add squash, butter and sage. Stir, adding pasta water as necessary to moisten. Serve immediately, topping with Parmesan.