In a bowl, stir together the crab,green onion, lemon juice, mayonnaise and mustard. Season well with salt and pepper.
Preheat broiler. Place the bread slices on a foil covered baking sheet and toast one side of the bread under the broiler. Turn over the slices and top evenly with the crab mixture. Sprinkle evenly with the cheese and broil just until the cheese melts.