How to Make It
Heat oven to 350° F. Heat the oil in a large skillet over medium heat. Add the leeks and cook, stirring frequently, until beginning to soften, 3 to 4 minutes. Add the mushrooms, ½ teaspoon salt, and ¼ teaspoon pepper and cook, tossing frequently, until the mushrooms are tender, 3 to 4 minutes more. Transfer to a bowl and let cool slightly.
Whisk together the eggs, half-and-half, and ¼ teaspoon each salt and pepper in a large bowl. Add the bread, Gruyère, and mushroom mixture and mix to combine.
Transfer to an 8-inch or other 2-quart baking dish and cover with foil. Bake until the edges are set but the center is slightly wobbly, 20 to 30 minutes. Uncover and bake until browned, 20 to 25 minutes more. Sprinkle with the chives.