Add dry ingredients and butter to a food processor and pulse until the mixture has formed a coarse crumb. Add water and egg and pulse until dough just comes together. Using two sheets of wax or parchment paper, form the dough into a disc and roll it out between the two sheets using a rolling pin or wine bottle until it will fit into a tart pan or pie dish. Transfer the sheet of dough onto a plate or cookie sheet and place in the freezer for 10 minutes.
While the dough is chilling, slice the tomatoes into 1/2" wedges and lay out on a rimmed baking sheet. Season generously with salt and allow to sit at room temperature while you work on the tart shell.
Remove the sheet of shortcrust from the freezer and work the dough into the pie dish or tart pan. Trim the edges as necessary and save any remaining dough for patching any tears that may have occurred. Prick the bottom and sides of the tart shell with a fork and place back in the freezer for another 20 minutes.
Preheat the oven to 375°F.
While the tart is chilling for the second time, prepare the herb puree. Rinse out your food processor and add herbs, salt, and garlic and pulse until the herbs are finely cut. Add olive oil and pulse again until the mixture forms a paste. Set aside.
Pull the chilled tart shell out of the freezer. Line with parchment paper and add dried beans, rice, or pie weights until they climb up at least half the height of your tart shell walls. Bake for 20-25 minutes, or until crust has begun to solidify. Remove pie weights and parchment, and bake another 5-10 minutes, or until the bottom of the tart is no longer shiny. Allow the tart shell to cool to room temperature.
Blot the tomatoes with paper towels to remove excess moisture. Spread mustard in the bottom of the ambient temperature tart shell. Sprinkle grated cheese over the mustard. Add an even layer of herb puree over the cheese, then arrange the tomato slices on top of that. Keep in mind they will shrink in the oven, so be generous and really load the tart with tomatoes. Crack pepper over the top layer of the tart.
Bake for 50 minutes to an hour, or until the tomatoes are nicely roasted. Allow the tart to cool slightly. Best served warm. Keeps in the fridge up to 4 days.