Leeks and spinach are a great combination to use in a frittata. With the addition of thinly sliced potatoes and cheese, this frittata is a great midweek supper or a welcome dish at a ladies’ lunch. It’s also the perfect vehicle for using up leftover vegetables—it can be an exploration of flavors every time you make it, as mixing ingredients is part of the fun. I like to use two or three vegetables, as I think more muddles the flavors, and I often add potatoes as a nod to the Spanish tortilla de patatas, though you certainly don’t have to. Use a melty cheese, like Swiss or mozzarella. Serve the frittata in the pan at the table, in keeping with its rustic looks, or flip it onto a decorative dish. I adore frittatas served with a dollop of pesto, slices of warm crusty bread, a crisp green salad, and a …