Mac And Cheese With Pesto, Proscuitto, And Peas

Ingredients

• 4 tablespoons (1/2 stick) unsalted butter, plus a little more for the pan
• 1 quart whole milk
• 1 pound penne rigate
• 1/4 cup all-purpose flour
• 1 1/2 teaspoons salt
• 1/2 teaspoon freshly ground black pepper
• 2 teaspoons dijon mustard
• 3/4 cup frozen peas, defrosted
• 1 pound sharp white cheddar cheese, grated (4 cups)
• 1/2 pound mozzarella (fresh or supermarket-style), grated (1 cup)
• 6 slices prosciutto, cut into skinny strips
• 1/2 cup pesto, preferably homemade