Arrange tomato slices in a single layer on a baking sheet. In a small bowl, mix together mayonnaise, parmesan, Dijon mustard, and wholegrain mustard. Season with oregano, and salt and pepper to taste. Use a spoon to top each tomato slice with a small amount of the mayonnaise mixture. Sprinkle with mozzarella and then top with parsley.
Bake in preheated oven for approximately 15 minutes, or until golden brown. Serve immediately.