If making your own crust, line a tart pan with a removable bottom with the pie dough, and par-bake it following these instructions. While the crust is baking, core and slice the tomatoes ¼" thick. Lightly salt, and let them drain on paper towels or a cooling tray nested in a baking sheet. Make a pesto using the basil, garlic, parsley, salt, pepper and olive oil. (See below) Drizzle the pesto over the tomatoes.
Making the pesto: The most flavorful way to make pesto is to use a mortar and pestle. The quickest most flavorful way is using a mini-chopper. Add the basil, garlic, parsley, salt and pepper into the mini-chopper and chop it to a fairly fine consistency. Add the olive oil, and whirl again to mix it. A food processor can be used in a pinch, but it tends to create an inconsistent texture, with some basil pieces much larger than others.
When the crust is finished par-baking, lower the oven temperature to 375˚F. Cover the bottom of the crust with most of the ¼" thick slices of mozzarella, retaining a couple of slices. If needed, slice the mozzarella pieces to fit, instead of overlapping them. The cheese creates a barrier between the crust and the tomatoes, preventing the crust from becoming soggy. Layer the tomatoes on top of the cheese, overlapping them.
Bake for 15 minutes. Remove the tart from the oven and tuck the remaining slices into the top of the tart, between the tomatoes. Bake for another 15 minutes, or until the cheese is melted and the tomatoes begin to give off liquid.
Cool on a cooling rack for 15 minutes before slicing into individual appetizer squares. Serve with sturdy toothpicks.