Tomato Gratin

Directions

1. 1
   Preheat the oven to 425.
2. 2
   Oil an 8-by-12-inch baking dish.
3. 3
   In a medium skillet, heat the olive oil.
4. 4
   Add the onion and thyme and cook over moderate heat, stirring occasionally, until the onion is softened, about 7 minutes.
5. 5
   Scrape the onion into the prepared baking dish and spread evenly over the bottom.
6. 6
   Arrange the tomato slices over the onion in overlapping rows.
7. 7
   Season with salt and pepper and drizzle with the balsamic vinegar.
8. 8
   Dot the tomatoes with the butter and sprinkle the Parmigiano-Reggiano on top.
9. 9
   Bake on the upper rack for about 15 minutes, until the cheese starts to brown and the gratin is bubbling.
10. 10
    Let rest for about 5 minutes before serving.