At first glance this looks like a simple frittata. You're close. There are certainly eggs in this dish, but not nearly as many as you'd imagine. In fact, this recipe from Mollie Katzen's Get Cooking is a popover, which uses a quick batter that is blended up and then poured over sautéed mushrooms. After 25 minutes in the oven, it puffs up slightly in the middle, acquiring a texture that is light and airy, not eggy and dense. Meanwhile, the edges get golden brown and crunchy, which is never a bad combo.
With the eggs playing a slightly less important role, the mushrooms take the lead in flavor. Katzen recommends using two different varieties, white and shiitake mushrooms. The earthiness of the latter is especially apparent. Some nice woody herbs, thyme and rosemary, help round things out. Also, you should note that the recipe states that the eggs should be at room temperature, so that they will rise more during the baking process.