Prepare the Pie Crust: Preheat your oven to 450°F. Unroll the store-bought pie crust and place it into a 9-inch pie dish, pressing it into the bottom and up the sides. Crimp the edges if desired. Use a fork to poke holes in the bottom of the crust several times to prevent it from puffing up while baking. Par bake the crust for about 10 minutes, or until it starts to lightly brown. Remove from the oven and let it cool slightly. Lower the oven temp to 350°F.
Prepare the Tomatoes: While the crust is baking, slice the tomatoes. Lay them out on a paper towel or clean kitchen towel and sprinkle lightly with salt. This helps to draw out excess moisture from the tomatoes. Let them sit for about 10 minutes, then blot the tops with another paper towel to remove the moisture.
Prepare the Filling: In a small bowl, mix together the mayo, sour cream, garlic powder, salt, and pepper, ¼ cup of parmesan cheese and cheddar cheese until combined.
Assemble the Pie: Spread the mustard evenly over the bottom of the pre-baked pie crust. Layer half of the tomato slices evenly on top of the mustard mixture. Next pour in the cheese mixture and spread out evenly inside the pie crust.
Top and Bake: Layer on the remaining tomatoes then sprinkle the monterey cheese and chopped parsley evenly over the top of the pie. Cover with foil then bake the pie in the preheated oven for 20 minutes, then uncover and bake for an additional 10 minutes or until the cheese is melted and bubbly, and the crust is golden brown. (Continue by broiling on high for 2-3 minutes if you prefer a bubbly, charred topping)
Cool and Serve: Once baked, remove the pie from the oven and allow it to cool slightly for about 10 minutes. Slice and serve warm, garnished with additional fresh parsley if desired.