Preheat the broiler to high.
Butter the halved muffins, place them in a single layer on a baking sheet and broil until golden, about 2 minutes.
Meanwhile, make the tuna salad: In a large bowl, mix together the tuna, artichoke hearts, mayonnaise, olives, mustard, dill, hot sauce, celery, scallions and lemon zest. Season with salt and pepper.
Top each muffin half with a large scoop of tuna salad, arrange 2 slices of cheese on each and broil until the cheese is melted, about 2 minutes. Remove and top with tomato slices and chives to serve.