Preparation Preheat oven to 350°F. Toast pistachios on a rimmed baking sheet, tossing once, until golden brown, 5–8 minutes. Let cool, then coarsely crush with a measuring cup or glass. Cook pasta in a large pot of boiling salted water 6 minutes, then add peas and cook until pasta is al dente, about 2 minutes more. Meanwhile, heat butter and 2 Tbsp. oil in a medium heavy pot over medium. Cook leeks, garlic, red pepper flakes, and 3 mint sprigs, stirring occasionally, until leeks are soft but not browned, 6–8 minutes; season with salt. Add buttermilk; bring to a simmer. Pluck out mint; discard. Using a slotted spoon, transfer pasta and peas to leek mixture. Add 3 oz. Parmesan; cook, stirring vigorously and adding a little pasta water if needed, until sauce is creamy and coats pasta, about 1 minute. Season with salt and pepper. Remove from heat; add lemon juice. Pull leaves from remaining mint sprigs, tear into pasta, and toss. Divide pasta among bowls. Top with pistachios and Parmesan; drizzle with oil.