Position your oven racks so that one is in the center. Preheat to 375 degrees.
Put the tomato slices in a colander, and place it in the sink. Let the tomatoes sit for 15 minutes to drain off any excess liquid.
Spread the mustard evenly over the par-baked tart shell. Sprinkle the cheese over the mustard, and sprinkle the herbs de Provence over the cheese.
Working from the outside in, lay the drained tomato slices in overlapping concentric circles, making sure the crust is covered entirely.
Crack the eggs in a small bowl. Whisk in the cream, salt, and pepper. This is your custard. Pour the custard evenly over the tomatoes until it comes to about 1/4 inch from the top edge of the crust. (If you have a little extra, don't worry about it).
Place the tart on the center rack in the oven, and bake for 45 mins, or until the custard is set. (Set custard won't jiggle when you shake the pan and will be firm when you touch it). The tomatoes in this tart may give off a hot liquid--don't confuse this with uncooked eggs and overcook your tart. The liquid will evaporate as the tart cools.
Remove the tart from the oven and set it on a wire rack. Allow the tart to cool slightly.