Heat the oven to 375°F. Lightly coat a 9-inch pie plate (not deep dish) with cooking spray, oil, or butter. Prepare any mix-ins while the oven is heating.
Whisk 5 large eggs, 1 cup whole milk, ¼ to ½ teaspoon kosher salt, and ⅛ teaspoon black pepper together in a large bowl until no streaks of egg remain.
Stir in the mix-ins (pick up to 3). Pour into the pie plate and arrange the mix-ins into an even layer.
Bake until puffed around the edges and a tester inserted in the center comes out clean, about 30 minutes.
