1. Heat olive oil in a large skillet over medium-low heat. Add onions, 1/2 teaspoon salt, and 1/8 teaspoon pepper; cook until soft and caramelized, stirring occasionally, about 30 minutes. Remove from heat and set aside.

2. Heat oven to 350 degrees. Meanwhile, in a small saucepan, heat stock and saffron over low heat until the stock turns yellow and is infused with saffron, about 3 minutes. Remove from heat and set aside.

3. Combine potatoes, garlic, oregano, and thyme in a large bowl and add the saffron-infused stock. Gently toss with a rubber spatula to combine well.

4. Spray a 12-inch gratin dish with vegetable-oil spray and arrange half the potato mixture evenly in the dish. Sprinkle with 1/2 teaspoon salt and 1/8 teaspoon pepper and spread the caramelized onions over potatoes. Arrange tomato slices on top of the onions and season with 1/2 teaspoon salt and 1/8 teaspoon pepper. Layer remaining half of the potatoes over tomatoes and sprinkle with remaining 1/2 teaspoon salt and remaining 1/8 teaspoon pepper. If any stock remains in the bowl, pour it over the gratin.

5. Cover gratin with aluminum foil and bake for 1 hour. Remove foil, baste with juices from pan, sprinkle with Parmesan, and bake, uncovered, until the top is golden brown and crisp, about 20 to 25 minutes more. Let stand for about 15 minutes before serving. This dish can be made up to 4 hours in advance and served at room temperature or reheated in a 300 degrees oven for 15 minutes.