Semolina crust:
In a food processor, combine flour, semolina, thyme, and salt.
Add butter and mix, stopping and starting the machine, until the mixture has a coarse texture.
Without stopping the machine, add ice water and lemon juice all at once, and mix until the dough begins to form.
Shape the dough into a ball and flatten it into a disk.
Wrap the disk in plastic wrap and refrigerate for 30 minutes.
The wrapped dough can be stored in the refrigerator for up to 2 days. Let it come to room temperature before proceeding with the recipe.
On a floured surface, roll out the dough and place it in a 9-inch (23 cm) diameter tart mold with a removable bottom, greased with butter.
Prick the dough with a fork all over the surface.
Refrigerate for 30 minutes.
Bake in the lower third of the oven preheated to 400 F (200 C) for 18 to 20 minutes, or until the crust is golden brown.
Let the tart cool on a rack.
For the topping:
Meanwhile, in a glass dish that can go in the oven, 13 x 9 inches (33 x 23 cm), combine tomatoes, olive oil, garlic, salt, and pepper.
Bake in the preheated oven at 400 F (200 C) for 30 minutes, or until the tomatoes are slightly charred and wrinkled.
Brush the cooled crust with Dijon mustard.
Crumble half of the feta cheese into the crust.
Cover with roasted tomatoes and sprinkle with the remaining crumbled feta cheese.
Bake in the center of the oven preheated to 400 F (200 C) for 15 minutes, or until the topping is warm.
Serve warm or at room temperature.