3 pounds ripe plum tomatoes, halved lengthwise and seeded
1/4 cup extra-virgin olive oil
1 tablespoon thyme leaves
Kosher salt and freshly ground pepper
2 garlic cloves, thinly sliced
About 1 cup all-purpose flour
6 1/2 tablespoons cold unsalted butter, cut into 1/2-inch pieces
1/4 cup ice water
2 tablespoons crème fraîche
1 tablespoon whole grain mustard
1/2 cup shredded Gruyère cheese
Preheat the oven to 350°. Toss the tomatoes with the olive oil and thyme; season with salt and pepper. Spread the tomatoes, cut side down, on a rimmed baking sheet. Roast for 35 minutes. Pull off the tomato skins. Turn the tomatoes cut side up, top with the garlic and roast for 35 minutes longer, or until slightly dried and the garlic is golden. Let the tomatoes cool, then blot dry with paper towels. Leave the oven on.
Meanwhile, in a food processor, combine 1 cup of flour with a pinch of salt. Add the butter and pulse until it is the size of small peas. Sprinkle on the ice water and pulse just until a dough forms. Wrap the dough in plastic wrap and refrigerate until chilled, about 30 minutes.
On a lightly floured surface, roll out the dough to an 11 1/2-inch round about 1/8 inch thick; fit it into a 9 1/2-inch tart pan with a removable bottom. Fold in the overhang to reinforce the sides. Trim off any excess dough. Chill the tart shell.
Line the tart shell with foil and fill with pie weights. Bake the tart shell for 35 minutes, or until just set. Carefully remove the foil and weights and bake for 5 minutes longer, or until golden.
Mix the crème fraîche and mustard and spread over the tart shell. Sprinkle the cheese on top. Arrange the tomatoes in the shell in 2 layers, cut side up, seasoning between the layers. Bake the tart for 25 minutes, or until the tomatoes are just beginning to brown. Serve hot or at room temperature.