Set an oven rack in the middle position. Preheat the oven to 450 degrees F. Line a baking sheet with parchment paper.
Spread the squash and onion on the sheet pan and drizzle with the olive oil. Season with 1 1/4 teaspoons of salt, a couple turns of black pepper, paprika, dried mustard and nutmeg and toss to coat. Roast until the squash is tender and the edges are starting to brown, begin checking at 20 minutes.
Bring a large pot of salted water to a boil and cook the pasta until tender but slightly toothsome, 6 to 7 minutes. Reserve 1 1/2 cups of pasta water and set aside. Drain pasta and return the pot to the stove.
Combine the squash, vegetable broth, and half of the pasta water in a high powered blender until super smooth. Transfer to the pot and slowly bring up to a simmer. Add the cheese, whisking over low to medium heat until completely melted, 1 to 2 minutes. Thin out with additional pasta water if it is too thick. Add the pasta and toss to coat. Season to taste with salt and pepper. Serve with lots of Sriracha on top.
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