Preheat oven to 375 degrees.
Prep 8 x 13 inch baking pan by lightly greasing the pan.
Add crabmeat in a large bowl. Drizzle the lemon juice on the crab. Stir in the panko bread crumbs.
Add melted butter,⅕ cup parmesan cheese, dijon mustard, horseradish, garlic and onion powder. In with the crab mixture.
Scoop out the crabmeat mixture with a cookie scoop.
Fill the mushroom caps with the mixture.
Sprinkle the remaining parmesan cheese on top of the filled mushrooms.
Bake at 375 degrees until the mushrooms become tender. (About 15-20 minutes).
Take out of the oven and serve immediately.
Enjoy!