These were created using baby bella mushrooms, if you are using large portobello I would recommend removing the gills before stuffing, the earthiness of the mushroom with the gills is too overpowering for the delicate crabmeat. My family prefers parsley as the herb, but you can add any other of your choice such as dill.
Remove caps from cleaned mushrooms and chop finely. Melt 1 T. butter in sauce pan and sauté the chopped mushroom caps with the peppers and onions until slightly soft, 2 to 3 minutes. Remove from heat and cool 3 to 5 minutes.
Into a large mixing bowl, combine sautéed vegetables with the fresh bread crumbs, parsley, Old Bay, horseradish, lemon juice, egg and 1/4 cup cheese. Add crabmeat, and gently combine.
Generously butter a baking dish large enough to hold your stuffed mushroom caps. Pour 2 T. melted butter into dish. Fill each cap with a heaping mound of the crabmeat mixture and place into the baking dish. Add the wine, top the filled caps with the remaining cheese and the panko crumbs.
Bake at 400, loosely covered with foil, for 15 minutes. Remove foil and continue cooking until cheese has melted and topping is lightly browned, 10 to 15 minutes depending on the size mushroom you use.