Preheat the oven to 400F.

In a large skillet over low heat, add the butter and the onion with ½ teaspoon of salt. Let the onion caramelize for about an hour, maybe longer, stirring every 5 minutes or so while you make the rest of the filling.

Place the pie crust in the preheated oven to parbake for 15 minutes. Remove the crust from the oven and reduce the heat to 375F.

While the onion is working, dice all but one or two of the tomatoes. Slice the remaining tomatoes - just enough to top the pie at the very end. Place the sliced tomatoes on paper towels to dry slightly.

Separate the remaining tomatoes in half. Place half of the tomatoes in a colander over a bowl with ½ teaspoon salt and ½ teaspoon sugar. Let drain as much as possible while you prepare the rest of the pie.

Place the remaining diced tomatoes on a baking sheet with ½ teaspoon salt and the olive oil. Roast for 30-40 minutes until dried slightly and browned at the edges.

Make the cheese mixture by combining the mayo, cheeses, and a few pinches of salt and pepper in a bowl. Set aside.

When the filling components are done (crust is parbaked, sliced tomatoes have drained, diced tomatoes have drained and roasted, onion is caramelized), finish the filling and the pie. Combine both sets of diced tomatoes with the caramelized onions and the remaining ½ teaspoon of salt and sugar in a bowl with a few grinds of pepper. Taste, and add more salt, sugar, or pepper if you'd like.

Spoon the tomato/onion mixture into the pie crust and top with dollops of the cheese mixture. Arrange the sliced tomatoes all over the top of the pie and sprinkle them with salt and pepper.

Bake the pie for 30 minutes until the crust is brown and the filling is set. Let the pie cool for 10 minutes or more before slicing and serving.

It's worth it, I promise.