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If your pistachios are not already roasted, heat an oven to 350° and roast them for 5-8 minutes. Coarsely crush using a glass or bottom of a pan.
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If you are using the Italian sausage, cut the casing off of the two links and saute in small skillet until done. Drain off fat.
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Cook the pasta in a large pot of boiling salted water for about 6 minutes. Add the sugar snap peas for the last two minutes of cooking time.
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While the pasta is cooking -- put the 2 tablespoons of oil in a medium heavy pot and cook over medium heat the leeks, garlic, red pepper flakes. Add in the basil when these have sautéed for a few minutes. Add in the salt and pepper. Stir in the buttermilk and bring to a simmer.
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Using a slotted spoon drain the pasta and peas and put into the leek mixture. Add 3 ounces of parmesan cheese, cook, stirring to combine. Add in a little pasta water if needed until sauce is creamy and coats the pasta. This takes about 1 minute. Remove from heat and stir in the lemon juice. Put pasta in serving bowls and top with pistachios and parmesan; drizzle with some olive oil.