Place the tomatoes in a 9 x 13-inch ceramic dish. Add the 1 1/2 tablespoons of olive oil, thyme, 1 teaspoon salt, and 1/2 teaspoon pepper and toss together. Spread the tomatoes evenly in the pan.
Place the garlic, parsley, and 1/2 teaspoon salt in the bowl of a food processor fitted with the steel blade and process until the garlic is finely chopped. Add the bread cubes and process until the bread is in crumbs. Add the 1/4 cup of olive oil and pulse a few times to blend. Sprinkle the crumbs evenly over the tomatoes.
Bake the gratin for 40 to 45 minutes, until the crumbs are golden and the tomato juices are bubbling. Serve hot or warm.