Preparation

• Lightly brown the bacon in a skillet over medium heat, about 4 minutes. Pour off all but 2 Tbs. of the fat. Add the onion and cook over low heat until caramelized, about 20 minutes. Meanwhile, combine the Parmigiano-Reggiano, breadcrumbs, and oil and set aside.
• Heat the oven to 350°F. On one side of a round or oval shallow baking dish, arrange 4 or 5 tomato slices, overlapping them slightly. Spoon some of the bacon-onion mixture on the lower half of each tomato slice, and then lay another row of tomatoes across the first row. Layer the tomatoes at a slight angle against one another, topping each slice with the bacon-onion mixture. Repeat until the dish is full. Sprinkle with the salt, pepper, fresh thyme, and the Parmigiano-breadcrumb mixture.
  
• Cover the dish with foil and cook for 30 minutes. Remove the foil and continue cooking until the tomatoes are very tender and the top of the gratin is browned, about another 30 minutes.