Preheat oven to 275°F. Arrange tomato halves, cut side up, on a baking sheet. Scatter thyme sprigs and garlic on top of tomatoes. Season with salt and pepper and drizzle with 3 Tbs. oil. Roast until tomatoes have dried slightly and some of the skins have burst, about 2 hr.

Meanwhile, in a small bowl, using a rubber spatula, stir together mascarpone and Parmigiano-Reggiano cheeses until smooth; refrigerate.

Pass roasted tomatoes through a tomato press according to manufacturer's instructions. In a small stockpot over medium heat, warm 2 Tbs. oil. Add onion and saute, stirring occasionally, until tender and translucent, 4-6 min. Add pureed tomatoes, stock and bread and simmer for 5 min.

Using a stick blender, puree soup and season with salt and pepper. Divide soup among 6 warmed soup bowls and top each with a dollop of the cheese mixture.