In a large bowl combine the tuna, cucumber, onion, avocado, cilantro, mayo, lemon juice, salt and pepper. Stir to combine. Scoop about 1 tablespoon of the tuna mixture onto each cracker and place the crackers on a baking sheet.
Top each stack of tuna with a slice of tomato and a sprinkle of cheese. Bake for 3 to 4 minutes, until cheese is melted. Serve warm and enjoy!