Preheat oven to 375 degrees F. Remove stems from mushrooms and place the caps into a lightly greased 9x13 pan.
Place lump crab meat into a large mixing bowl. Pour lemon juice over the top. Add in panko bread crumbs and stir together. Pour melted butter over mixture and add in parmesan cheese, dijon mustard, horseradish, garlic powder, and onion powder.
Scoop spoonfuls of crab mixture and place into the mushroom caps to fill all of them.
Bake in the preheated 375 degree oven for 15-20 minutes, or until mushrooms are tender. Remove from oven and serve while hot. Garnish with chopped parsley, if desired.