Heat the oven to 350 F and spray a small casserole dish with non-stick spray.
In a medium bowl combine the cream cheese, mayonnaise, lemon juice, Old Bay, and mustard powder until smooth.
In a pan over medium heat melt the butter. When it begins to foam add the onions and bell pepper. Season with salt and pepper. Cook until soft, about three to five minutes. Add the garlic and cook for thirty seconds.
Pour the vegetable mixture into the cream cheese mixture and mix well. Add the crab and fold in gently.
Pour into the prepared dish and top with the panko bread crumbs and parmesan cheese.
Bake for 45 to 55 minutes, or until golden brown and bubbly. Allow to cool for five minutes before serving.