Combine mayonnaise, dill and celery seed in a small bowl. Toast the slices of bread. Spread each piece of bread with about 1 tablespoon of the mayonnaise.
For each sandwich, place 3 ounces of ham and one slice of cheese on a slice of bread.
Grill in a toaster oven (or in a conventional oven) until the cheese is melted -- about 3 to 5 minutes. Remove sandwich from the oven and top with a slice of tomato and another slice of bread. Makes 4 sandwiches.