Sprinkle a layer of mozzarella cheese on the bottom of the pie shell. Slice the tomatoes 1/4" thick. Add a layer of sliced tomatoes on top of the cheese. Spread them with 1 Tbsp mayonnaise. Add a layer of Gruyere, a sprinkling of dried oregano, salt and pepper, a second layer of tomatoes. Spread with the final tablespoon of mayonnaise, a dusting of oregano, salt and pepper, and half of the remaining mozzarella cheese. Top with the final layer of tomatoes, oregano, salt and pepper, and the remaining mozzarella.