In a large pot of generously salted water, cook the pasta until al dente according to package instructions. Drain, reserving 1/4 cup of pasta water. Rinse pasta with a bit of cold water or toss with a teaspoon of olive oil to prevent sticking while preparing the other ingredients.
Meanwhile, steam the butternut squash until fork tender, around 10 -15 minutes. (Note: larger pieces will take longer to cook). Alternately, you can cook the squash in boiling water for approximately the same amount of time if you don’t have a steamer. After boiling, drain through a colander, shaking to remove excess water.
In a Dutch oven or heavy-bottom saucepan set over medium-low heat (you can use the same pot used for the pasta), add 2 teaspoons olive oil and a pinch of salt. Once the oil is heated, add the onions. Cook for 5-7 minutes, until the onions are translucent or just starting to brown. If necessary, add a tablespoon of water to the pot periodically to detach any brown bits that form, stirring them back into the onions. Add the garlic and sage and cook for an additional minute, stirring frequently, then remove the pot from the heat.
Microwave the half-and-half for around 45-60 seconds until warm. If you prefer, you can add the half-and-half directly to the pot with the onions and stir over low heat until warmed through, but be careful to make sure the liquid doesn’t bubble and reduce.
Puree the butternut squash, onion mixture, parmesan cheese, half-and-half, salt, pepper and nutmeg in blender. Taste and adjust seasoning if needed.
Toss sauce with the pasta. If the sauce is too thick for your liking, you can add some of the reserved pasta water.
Serve immediately, optionally garnishing each bowl with a small amount of chopped sage. Store leftovers in an airtight container in the refrigerator and enjoy within 3-4 days.
For a lighter meal, you can replace the half-and-half with whole milk or part-skim milk. For the cheese, I recommend using a microplane zester or the finest setting on your box grater. Larger shreds of parmesan may not melt as well into the sauce, creating a lumpy consistency.