Preheat the broiler, setting the temperature to high. Set an oven rack in the middle of the oven. Line a rimmed broiler-safe baking sheet with foil.
Wipe the mushrooms clean with a damp paper towel. Gently twist off or cut the stem of each mushroom. Holding each mushroom in one hand, use a spoon to gently scrape out the gills.
Spray the mushroom caps with olive oil cooking spray on both sides. Sprinkle with ¼ teaspoon kosher salt, ⅛ teaspoon pepper, and ¼ teaspoon garlic powder. Broil until just tender, about 5 minutes on each side.
Remove the mushrooms from the oven. Drain any liquids. Switch the oven from broil to bake, setting the temperature to 400 degrees F.
Break an egg into each mushroom. If your eggs are large, you'll probably need to break each egg into a small bowl and pour some of the egg white out before pouring it into the mushrooms.
Sprinkle the mushrooms with the cheese. Bake until the egg whites are cooked, about 15 minutes.
Sprinkle the eggs with the remaining ¼ teaspoon salt and ⅛ teaspoon pepper. Garnish with parsley and serve.
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Notes
The FDA recommends cooking eggs thoroughly.During the final step of baking, bake just until the egg whites are cooked. Baking too long will result in mushy mushrooms that have released their liquid into the eggs. But even if that happens, do not despair: carefully drain the liquid, and place the mushrooms on paper towels to soak as much of the liquid as you can.