Preheat your oven to 375 degrees F. Spray a rimmed baking sheet with cooking spray.
Remove stems from mushrooms. Gently spoon out gills and base of the stem from mushrooms and discard.*
Melt butter over medium high heat in a saucepan. Add onions and cook (stirring frequently) for 3 minutes, or until tender. Add garlic and cook (while stirring) until fragrant (about 30 seconds).
Remove from heat. Add cream cheese, bread crumbs, Italian seasoning, Old Bay seasoning, salt and pepper. Stir until cheese is melted and mixture is smooth. Fold in crab meat.
Place mushroom caps onto prepared baking sheet. Spoon crab mixture into caps. Sprinkle evenly with Parmesan cheese.
Bake for 15 to 20 minutes, or until heated thoroughly.
Sprinkle with chopped fresh parsley, if desired.