Preheat the oven to 375 degrees.
Using a small paring knife, remove the stems of the mushrooms if they are intact. Then using a spoon, scoop out the gilled side until you see white mushroom underneath. Discard the gills and stems.
Place the mushroom caps in a large baking dish, such as a pyrex glass casserole dish. They should fit snug.
In a medium bowl combine the diced celery, diced roasted red bell pepper, crab meat, mayonnaise, all seasonings, and lemon juice. Fold the ingredient in the mixing bowl with a spatula to combine and taste for seasoning.
Fill each mushroom cap to overflowing.
For the brie, remove the rind and slice it into manageable slices.
Place the slices on top of the mushrooms.
Place 2 tablespoons of sliced butter into the dish. Sprinkle the whole dish with sea salt and garlic powder, and cover with foil.
Bake at 375 for 30 minutes. You may remove the foil and bake for an additional 5-10 minutes to melt and brown the cheese if desired.
Remove the mushrooms and sprinkle with extra lemon juice and fresh parsley or green onions. Enjoy!