For a richer flavor, try using roasted garlic in the butter bean sauce. Add a pinch of red pepper flakes to the sauce for a touch of heat. To toast the walnuts, spread them on a baking sheet and bake at 350°F (175°C) for 5-7 minutes, or until fragrant and lightly browned. This pasta is best served immediately, but leftovers can be stored in an airtight container in the refrigerator for up to 2 days.