Sour Cream-and-Pickled Onion Sauce

From Test Kitchen Director Robby Melvin:Turn your ho-hum leftover turkey sandwich into a gourmet lunch with our Test Kitchen Director's go-to sandwich condiment. The best part? There's no shopping needed. This sauce can be made with ingredients found in your refrigerator. To build a spectacular sandwich, slather the sauce on toasted slices of your favorite bread. Then pile on the leftover turkey and top with thinly sliced red onion, crisp lettuce leaves, and a juicy slice of tomato.