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bring water to a boil.
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brown whole sausage in a dutch oven, until mostly cooked.
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remove the sausage from pan and slice into coins.
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cook pasta according to package directions—be sure not to overcook it; the pasta should be quite al dente.
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meanwhile, add butter to the dutch oven and cook over moderate heat about 5 minutes until beginning to color and separate.
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it should get quite nutty, but don’t let it burn!
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add sage leaves and garlic to the butter and cook for a minute to infuse flavors.
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return sausage to pan, toss until warm and add red pepper flakes if desired.
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when pasta is cooked, drain and add (along with a few tablespoons of pasta water), stir over low heat for one minute.
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add salt and pepper to taste, serve with parmesan.