When the crust comes out of the oven, lay the sliced cheese in the bottom of the crust; this will prevent it from becoming soggy. Spread half the onion in the bottom of the crust and arrange the tomato slices over it. (For extra cheesy goodness, you can crumble a bit of goat cheese and dot the tomatoes with it!) Sprinkle the salt, pepper and oregano over the tomatoes. Layer onion, tomatoes, and spices until your pie is full.