Preheat the oven to 350°F (175°C) and spray a large baking sheet with non-stick spray. Set aside.
Before removing the stems, clean the mushrooms by gently wiping the mushroom caps with a damp paper towel to remove the dirt. Once all the mushrooms have been cleaned, carefully remove the stems and scoop out the gills.
Precook the mushrooms (Optional): Before stuffing, place the cleaned mushroom caps stem-side up on your prepared baking sheet and bake for 3-4 minutes. Remove from the oven and flip each mushroom cap over to drain any excess moisture.
In a medium bowl, combine the crab, 3 tablespoons of melted butter, green onions, 5 tablespoons of breadcrumbs, grated Parmesan cheese, mayonnaise, egg, and seasonings (dried Italian seasoning, garlic powder, salt, and red chili flakes). Mix well to combine.
In a separate, smaller bowl, combine the remaining 3 tablespoons of breadcrumbs, 1 tablespoon of butter, and fresh parsley. Set aside.
Fill each mushroom cap with approximately one tablespoon of the crab filling (I like to use a cookie scoop to help with this part) and place them on the prepared baking sheet. Sprinkle the breadcrumb topping evenly over each mushroom.
Bake for 20 minutes, rotating the pan at the halfway mark. The tops should be golden, and the mushrooms should be cooked. If desired, garnish with freshly minced parsley and fresh lemon juice.