Saute zucchini in a single layer in a saute pan with a little bit of oil until they begin to brown, then turn over and brown the other side a little (this is to both heat them and eliminate a little of the moisture in them).
Mix drained tuna with parsley, olives, green onions, mayonnaise and mustard (you can swap out ingredients here--try pickled red peppers, capers, celery, brown onion, swap out different peppers for greater or lesser heat--whatever you have on hand).
Arrange zucchini in small piles on a foil lined baking sheet: I used 4-5 pieces per pile and just slightly overlapped their edges.
Mound tuna onto zucchini bases.
Cover each tuna-zucchini mound with a few slices of tomato and then sprinkle with cheese.
Place under a broiler until cheese is melted and beginning to brown.