Start by making the crust. This is the fun part! Pre-heat the oven to 350°F.
Whisk together dry ingredients in a bowl until blended. Then, add wet ingredients and stir gently until the dough forms a ball. (For a tender crust, do not over mix).
Put dough into pie pan; it will be crumbly but that's okay. Pat it thin and flat to conform to the shape of the pan. Crimp the edges of the crust with your fingers to make it purty!
Bake the crust in a pre-heated oven for 15 minutes until it is golden brown. (You may want to cover the edges with aluminum foil to prevent scorching.)
Prepare the filling while the crust bakes up: Wash and slice your tomatoes, and let them drain on paper towels for about 10 minutes. Peel and slice the onion, place the slices on a glass plate, and microwave for 5 minutes - or saute in a little oil to soften (you don't want them to stay crunchy in the pie).
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.
In a small bowl, combine the mayonnaise with the Parmesan and Mozzarella cheeses. (You can also use Romano or Cheddar for a twist!) Spoon this mixture over the tomatoes and spread it out with the back of your spoon. If you like, you can sprinkle a little more oregano over the top.
Bake the tomato pie for 20 minutes until it is golden brown and delicious!