Heat the oven and prepare the baking dish. Arrange a rack in the middle of the oven and heat the oven to 375°F. Lightly coat a 9x13-inch baking dish with cooking spray, oil, or butter. Prepare any mix-ins while the oven is heating.
Whisk the eggs. Place 10 large eggs in a large bowl and whisk until fully beaten.
Whisk in the milk, salt, and pepper. Add 1 to 2 cups whole milk, 1 teaspoon kosher salt, and ¼ teaspoon black pepper. (If you want a sturdier, less custardy egg bake that can, for instance, get packed in a lunch box, use 1 cup or 1 ½ cups milk instead.) Whisk until frothy.
Add the mix-ins. Stir in any mix-ins such as meat, cheese, vegetables, or herbs (see above for amounts).
Bake for 45 minutes. Pour into the prepared baking dish and arrange the mix-ins in an even layer. Bake until puffed around the edges and a tester inserted in the center comes out clean, about 45 minutes.