Pick over the crabmeat, discarding any pieces of shell. Finely chop the meat. In a medium bowl, combine the crabmeat, scallions, celery, lemon juice, mayonnaise, salt, and pepper. Cover and refrigerate for 30 minutes. Meanwhile, toast the rolls until lightly browned. Spoon some of the chilled crabmeat mixture onto half of each roll, then place some of the lettuce and tomato on the other half. Serve immediately.Tip: You can substitute chopped lobster meat or peeled, deveined, and finely chopped shrimp for the crabmeat.