This recipe takes a handful of basic ingredients and turns them into a rich, satisfying breakfast, lunch or dinner. Each element plays an essential role. Crusty bread toasted in butter and olive oil gives the frittata substance and texture, onions cooked until sweet and caramelized add depth, cheese brings umami and gooeyness, and the eggs provide richness and tie everything together. Cooking starts on the stovetop but finishes at 425°F, so you will need a 10-inch nonstick skillet that’s oven-safe. Serve a crisp green salad alongside as a counterpoint to the frittata’s richness.