Preheat the oven and prep the mushrooms - Preheat the oven to 425℉, convection bake setting. Brush off the mushrooms and remove the stems. Rub the mushroom caps with olive oil and place the mushroom caps, gill-side up, on a lined baking tray. Sprinkle with a bit of salt, and bake for 10 minutes. Pull the mushrooms out of the oven, and pour out the liquid that has pooled in the caps. You can discard the liquid or save it to add to vegetable stock.
Prep the vegetables and the bread crumb topping - Mince the scallions and garlic. Mince the red bell pepper, if using. Mix 1/4 cup of the Parmesan with the bread crumbs.
Make the filling - Mix the cream cheese, mayo, lemon juice, scallions, bell pepper (optional) garlic, Old Bay, salt, pepper, and remaining 1/4 cup of Parmesan. Gently fold in the crab meat.
Assemble and bake - Spoon a dollop of filling on each mushroom cap, then top with the Parmesan bread crumb mixture. Bake for 10 minutes or so, until the bread crumb mixture is toasted and the stuffed mushrooms are warmed through.