Prepare the crust by transferring one pre-made pie dough to a 9-in. pie plate.
Flute the edges with your fingers. Place into the refrigerator for 30 minutes.
Preheat oven to 425 degrees. Line the crust with a double thickness of tin foil. Fill with pie weights, dried beans or uncooked rice.
Place into the oven and bake on a lower oven rack 20-25 minutes or until edges are golden brown.
Remove foil and weights; bake 3-6 minutes longer or until bottom is golden brown.
Cool completely on a wire rack.
Reduce the oven to 375 degrees.
Make the filling by tossing HALF of the diced tomatoes with 1⁄2 teaspoon salt and 1⁄2 teaspoon sugar.
Set them over a colander to drain in the sink while you get everything else ready, at least an hour.
Place the other half of the diced tomatoes on a baking sheet.
Toss with the olive oil and fresh thyme leaves then season with sea salt and freshly cracked black pepper, to taste; toss to coat evenly.
Place into the oven to roast for 25-30 minutes, dried and slightly browned.
Remove from the oven; set aside to cool.
Make the caramelized onions while the tomatoes are draining in the sink and the others are roasting in the oven.
Heat the butter in a large skillet over medium heat. Add the onions and cook, stirring occasionally, until deeply caramelized.
If the onion gets dry, add a couple tablespoons of of water to the pan, scrape up the over-browned bits, and continue cooking, stirring occasionally. This will take about 45 minutes.
Make the topping by combining the mayonnaise, Fontina, and Parmigiano-Reggiano cheese then season with freshly cracked pepper, to taste; mix well.
Prepare the pie by combining the drained tomatoes, roasted tomatoes, caramelized onion, and fresh basil in a bowl; mix until well combined.
Pour into the prepared and cooled pie crust.
Spoon the topping evenly over the filling.
Top with a few slices of heirloom tomatoes that you've patted very dry with paper towels.
Place into the oven and bake for 30 minutes, or until the top of the cheese is bubbling and starting to turn golden brown.
Remove from the oven and allow to cool. Top with fresh basil before slicing and serving. Enjoy!