Preheat the oven to 350 degrees Fahrenheit, and place racks in the top and bottom thirds of the oven.
In a large mixing bowl, toss the tomatoes with olive oil, thyme, and a generous pinch or two of salt. Arrange the tomatoes, cut side down, on a rimmed baking sheet, and pour over them any oil that is left in the bowl. Slide the tomatoes into the oven and bake for 35 minutes. Remove the tomatoes from the oven, and let them sit for about 5 minutes. Working carefully, remove the tomato skins. [I find that a pair of rubber gloves—the yellow, snug-fitting kind that you might wear when you wash dishes—helps here, but if your fingers are less heat-sensitive than mine, you might be just fine without them.] Turn the tomatoes cut side up, nestle the slices of garlic into their flesh, and roast for 30-35 minutes more, until the tomatoes look a little dry and the garlic is pale golden. Let the tomatoes cool.
While the tomatoes cook, roll out the pâte brisée on a lightly floured surface. Press it gently into a 9-inch tart pan with a removable bottom, and fold in the overhang to reinforce the sides. Trim away any excess dough. Chill the tart shell until the tomatoes are finished roasting.
When the tomatoes are ready, line the tart shell with aluminum foil and fill with pie weights. Bake the tart shell for 30-35 minutes, or until just set. Remove the foil and the weights, and bake the tart shell for 5 minutes longer, or until pale golden.
In a small bowl, mix the crème fraîche and the mustard; then spread the mixture evenly over the bottom of the tart shell. Sprinkle the cheese on top. Arrange the tomatoes in the tart shell in two layers, cut side up. Bake the tart for 25 minutes, or until the tomatoes are just beginning to brown at their edges. Serve hot, warm, or at room temperature.