Whisk the eggs, milk, pepper, and salt in a large bowl to blend well. Stir in the cheese and parsley. Arrange the mushrooms in the prepared ramekins and pour the egg mixture. Bake until the egg mixture puffs and is just set in the center, about 15 minutes. Using a rubber spatula, loosen the frittatas from the ramekins and slide the frittatas onto a platter.

Serve immediately.