Heat the oven to 350°F. In a 10-inch round gratin dish, arrange the squash slices and tomatoes in an overlapping pattern to fill the dish. Brush the vegetables with olive oil; drizzle the remaining oil on top. Drizzle the chicken stock on top. Sprinkle with salt and pepper and 1 tablespoon thyme leaves.
In a medium bowl, combine the bread crumbs, butter, and remaining thyme, and season with salt and pepper. Arrange the bread-crumb mixture on top of the vegetables.
Bake until the vegetables are tender and the bread crumbs are golden, about 50 minutes. Remove from the oven. Serve garnished with thyme sprigs.
