Preheat oven to 400 degrees. Spray a 13x9 inch glass casserole dish with nonstick cooking oil and set aside.
Rinse, dry and remove any extra woody stems from the mushrooms. Place mushrooms in prepared dish.
In a small bowl mix vinegar, egg yolk, garlic, onion powder, Italian seasoning and mustard, slowly add the oil in a thin stream until fully incorporated. Drizzle over mushrooms and stir to fully coat with marinade. Roast at 400 degrees for 15 minutes. Remove from oven and baste with the marinade in the pan.
Turn oven to low broil. Sprinkle mozzarella cheese over mushrooms and return to broil 2 - 3 minutes or until cheese is golden. Serve with a slotted spoon.