Preheat oven to 425ºF. Coat a 9 by 13 baking dish with cooking spray. Break stems off portobello mushrooms. Scrape gills out of them with a spoon. Wipe caps with a damp paper towel. Lay gilled side up in the dish. Set aside.
Beat cream cheese, Old Bay, salt,pepper, lemon juice, Worcestershire sauce and Dijon mustard with an electric mixer on medium-low until creamy and the seasonings are fully incorporated into the cream cheese. Add egg white and beat again to combine.
Add scallions, ½ c Panko breadcrumbs, ½ Parmesan and red pepper and mix on low until combined.
Stir crab, gently by hand into the cream cheese mixture until just incorporated.
Scoop the filling into the mushrooms, dividing evenly.
Stir the remaining 3 tablespoons panko with butter and paprika until the breadcrumbs are evenly moistened and reddish orange. Sprinkle over the mushrooms. Sprinkle with the remaining 1 tablespoon Parmesan.
Bake the mushrooms until they are soft, the filling is heated through and the topping is browned, 30 to 32 minutes. Let cool 5 to 10 minutes. Top with additional scallions before serving, if desired.
