How to Make It
Preheat oven to 350°F with racks in the middle and upper positions. Whisk eggs, corn, ½ cup cheese, ¾ teaspoon salt, and ½ teaspoon pepper in a medium bowl.
Heat oil in a large oven-safe skillet over medium. Add mushrooms, leek, and remaining ¾ teaspoon salt and ½ teaspoon pepper. Cook, stirring often, until leek begins to brown and liquid has evaporated, 12 to 15 minutes. Transfer half of the vegetables to a medium bowl.
Add egg mixture to skillet, tilting to distribute evenly. Cook, without stirring, until frittata begins to set around the edges, about 2 minutes. Transfer skillet to the middle rack of oven and bake until just set in the center, about 15 minutes. Remove from oven.
Increase oven temperature to broil. Scatter remaining 1½ cups cheese and reserved vegetables over frittata, all the way to edges of skillet. Broil until cheese is melted and browned in spots, 2 to 4 minutes. Let stand for 5 minutes before serving.