This recipe for potatoes o'brien is a wonderful way to use leftover boiled or baked potatoes. In fact, it's worth cooking a few extra potatoes just to have the leftovers so you can make this iconic comfort food. This variation on a traditional recipe for potatoes o'brien is called o'brien au gratin potatoes and is found in a slim cookbook called "Mrs. Rasmussen's book of one-arm cookery. " It wasn't that mrs. Rasmussen had only one arm, it's that one arm was already dedicated to holding a beer while she cooked.