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Place a large pot of salted water over high heat to boil. When water is boiling, drop tortellini and cook according to package directions. Drain cooked tortellini and reserve.
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Melt the butter, add the onion and garlic, and season with salt and pepper. Sauté until the onions soften.
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Stir in flour and cook about a minute. Whisk in the milk and cook until the liquid comes up to a bubble and the sauce thickens, 2-3 minutes.
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Stir in the nutmeg and pumpkin. Season with salt and pepper and simmer until thickened, about 1 minute. I found I needed quite a bit of salt.
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Toss the tortellini with the pumpkin sauce and transfer to a casserole dish. Cover with cheese and broil until cheese is browned.