1) Preheat oven to 375 degrees. Using a large, sharp knife, trim ends; halve squash crosswise to separate bulb from neck. Peel with a vegetable peeler. Cut both pieces in half lengthwise. With a spoon, scoop out seeds; discard.
2) Cut squash into 2-inch chunks; transfer to a small rimmed baking sheet. Toss with oil and sage; season generously with salt and pepper. Scatter garlic around squash. Roast until squash is very tender, about 40 minutes, tossing once halfway through. Remove and discard skin from garlic.
3) Transfer squash and garlic to a food processor; puree. With motor running, add half-and-half through the feed tube; process until smooth. Add 1 to 2 cups water; continue to process until smooth, adding water to thin if necessary. Season again generously with salt.
4) Cook pasta according to package instructions. Reserve 1 cup pasta water; drain pasta and return to pot. Pour sauce over pasta; toss to coat. Add some pasta water to thin sauce if necessary. Serve with desired toppings.
To Freeze: Cool sauce to room temperature; transfer to airtight containers, leaving 1 inch of space. Freeze up to 3 months. When ready to use, place plastic containers upside down under hot tap water to help release frozen blocks of sauce. Place blocks in a large saucepan; cover and reheat over medium-low, adding water to thin if necessary.
How much sauce?: This recipe makes about 4 cups sauce, enough for 3 lbs. cheese ravioli or 1 1/2 lbs. short pasta (8 servings). For 4 servings, use half the sauce (about 2 cups), and cook 1 1/2 lbs. cheese ravioli or 3/4 lb. short pasta.