In this episode, Martha offers a primer on this favorite starch. She begins with Scalloped Potatoes, also called Potatoes Gratin: very finely sliced potatoes cooked in milk and baked in a cream mix with cheese. Next comes a recipe that's ended up being associated with American food: Potato Salad. Then, in case your go-to mashed potato dish needs an upgrade, Martha reveals a few tricks to accomplish the ideal Puréed Potatoes. Finally she demonstrates how, in just…