Béchamel:

• 1 cup whole milk
• 2 tablespoons unsalted butter
• 2 tablespoons unbleached all-purpose flour
• ¼ teaspoon salt

Pasta:

• 2 tablespoons unsalted butter
• 1 tablespoon extra-virgin olive oil
• 2 medium leeks, chopped
• ¼ pound thinly sliced prosciutto, cut into thin strips
• Salt
• 1 pound fresh tagliatelle
• ½ cup chopped fresh chives

For the Béchamel: In a small saucepan, heat milk over medium-low heat; bring just to a boil and remove from heat. In a medium saucepan, melt butter over low heat. Add flour and cook, stirring constantly with a wooden spoon for 2 minutes. Do not brown. Add milk to flour mixture, stirring constantly until milk is incorporated. Cook, stirring frequently, especially along corners of pan, until sauce is the consistency of a thick cream, about 5 minutes. Stir in salt and remove from heat.

For the Pasta: Heat butter and oil in a large skillet over medium heat. Add leeks and cook, stirring occasionally until softened, about 4 minutes. Add prosciutto and béchamel and stir to combine. Remove from heat and cover to keep warm.

Bring a large saucepan of salted water to boil. Add pasta and cook until al dente. Drain, add to leek sauce and stir to combine. Add chives, toss and serve immediately.