Preheat broiler (I use a toaster oven).
In a bowl, combine crabmeat, scallions, celery, cheese, mayo, and Old Bay seasoning.
Slice tomato into at least 4 slices.
Place bread slices on a baking sheet and broil 3-4 inches from heat until lightly toasted then remove from oven.
Place 1-2 tomato slices on each slice of bread and top with crab mixture. Sprinkle with extra shredded cheese, if desired.
Return to oven and broil until cheese browns. (3-5 minutes).