Preheat 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 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 1/4 teaspoon kosher salt, 1/8 teaspoon pepper, and 1/4 teaspoon garlic powder. Broil 5 minutes on each side, or until just tender.
Remove the mushrooms from the oven. Drain any liquids. Switch oven from broil to bake, setting the temperature to 400 degrees F.
Break an egg into each mushroom. Sprinkle with the cheese. Bake 15 minutes, until the egg whites are cooked.
Sprinkle the eggs with the remaining 1/4 teaspoon salt and 1/8 teaspoon pepper. Garnish with parsley and serve.