Potato, Green Bean and Mushroom Gratin | Milk Street's Chris Kimball

Heat the oven to 400°F with a rack in the middle position. Mist a 9-by-13-inch baking dish with cooking spray and set aside. 

In a 12-inch skillet over medium, melt 4 tablespoons butter. Add the potatoes and ½ teaspoon salt, then stir to coat. Add ½ cup water, then distribute the potatoes evenly. Cover and cook until a skewer inserted into the potatoes meet no resistance, 15 to 20 minutes, stirring once about halfway through. Transfer the potatoes and any liquid in the pan to a large bowl, then use fork to mash to the consistency of chunky mashed potatoes; set aside. 

Wipe out the skillet and set it over medium. Add the oil, shallots and ¼ teaspoon salt. Cook, stirring occasionally, until the shallots are softened but not browned, 5 to 6 minutes. Add the beans and mushrooms, then increase the heat to medium-high. Cook, stirring occasionally, until most of the liquid released by the mushrooms has evaporated and the beans are bright green but still crisp, about 5 minutes; the vegetables should not begin to brown. Stir in the marjoram and cook until fragrant, about 30 seconds. Transfer to the bowl with the potatoes and cool for about 10 minutes. 

In a medium bowl, whisk the eggs, 2 ounces (1 cup) of Parmesan and ½ teaspoon each salt and pepper. Add the egg mixture to the vegetable mixture and fold with a silicone spatula until combined. Transfer to the prepared baking dish and spread evenly. 

In a small bowl, combine the panko, melted butter and the remaining 1 ounce (½ cup) Parmesan, then stir until the panko is evenly moistened. Sprinkle evenly over the vegetables. Bake until golden brown and bubbling at the edges, about 30 minutes. Cool for about 15 minutes before serving.