Butter for greasing dish
10 large eggs
1 cup whole milk
1 1/2 teaspoons salt
1/2 teaspoon black pepper
5 oz Swiss cheese, coarsely grated (1 1/2 cups)
10 oz cremini mushrooms, trimmed and coarsely chopped (1/4-inch pieces)
1 large sweet potato (8 to 10 oz), peeled and coarsely chopped (1/4-inch pieces)
1 large shallot, halved and thinly sliced
3 tablespoons olive oil
Whisk together eggs, milk, 3/4 teaspoon salt, and 1/4 teaspoon pepper until smooth, then whisk in cheese. Pour into baking dish. Bake in upper third of oven until puffed, golden, and set, about 20 minutes.
Meanwhile, toss together mushrooms, sweet potato, shallot, oil, and remaining 3/4 teaspoon salt and 1/4 teaspoon pepper in a 17- by 13- by 1-inch baking pan. Spread vegetables in an even layer, then roast in lower third of oven while eggs are baking, stirring twice after 10 minutes, until tender and golden brown, about 18 minutes.
Serve eggs with roasted vegetables spooned on top.