In bowl, combine tuna, mayonnaise, lemon juice and Dijon; stir in celery, green onion, dill and garlic. Spread mixture over toast; top evenly with tomato and cheese slices.
Preheat broiler. Place sandwiches on parchment paper–lined baking sheet; broil for 1 to 2 minutes or until cheese is melted. Tip: These sandwiches are also delicious with shredded cooked chicken or turkey.