In a mixing bowl, combine milk and breadcrumbs and set aside. In a small fry pan, heat the butter and saute the onions and bell peppers until tender. Remove from stove and let cool a bit.
With a small spoon, remove the gills of the Portobello mushrooms until they are smooth inside. Place them in an oiled baking dish. Pick through the crab making sure there are no shells. Put the onions, bell peppers, crab meat, parsley, Old Bay seasoning, baking powder, Worcestershire sauce and egg into the bowl with the bread crumbs and milk. Toss gently to combine leaving nice chunks of crab intact. Fill the Portobello mushrooms with the crab mixture. Loosely cover the baking dish with aluminum foil and bake for 35 minutes. Remove from the oven. Top each stuffed Portobello with chopped parsley and serve with a wedge of lemon.