In the summer, tomatoes need little more than salt to reach near perfection, but roasting tomatoes any time of year alongside sweet onions concentrates their sugars and elicits a surprising amount of umami. This fragrant combination serves as the essence of this frittata. But the true hero of this recipe, if you can believe it, is a humble cup of full-fat Greek yogurt, divided between the frittata mixture and a simple sauce. The yogurt works to loosen the structure of the frittata, leaving the baked center eggy, delicate and tangy, with a little extra sharpness from Cheddar cheese, and also serves as a sauce, whisked with mustard and the savory juices left on the baking sheet to ensure no drop of flavor is forsaken. The consequence is a rich and satisfying meal that’s perfect for breakfast or lunch.