Spinach Cake with Herb Salad

This cake—a cross between a custard and a frittata—tastes best at room temperature, so it’s best made several hours ahead of time. It’s great on a picnic, or as part of a cold lunch. It can be made with other greens, such as chard, mustard or turnip greens, or nettles. Made with spinach, the cake is a brilliant green, its flavor deeply satisfying.