Preheat oven to 450 degrees. Arrange plum tomatoes on a rimmed baking sheet. Drizzle with
olive oil, sprinkle with 1/4 teaspoon sugar, and season with salt and pepper. Roast tomatoes until edges begin to wrinkle and juices release, about 35 minutes.
Transfer tomatoes and juices to a bowl. Heat olive oil in a medium saucepan over medium heat. Add diced onion, ground coriander, ground cumin, and caraway seeds, and cook, stirring occasionally, until onions are tender, about 4 minutes. Add chiles or red-pepper flakes to saucepan. Add tomatoes and juices, and toss to coat. Add chicken stock, and bring to a boil. Reduce heat, and simmer,
about 10 minutes.
Filling a blender halfway and covering with a kitchen towel, puree soup in batches. Strain through a coarse sieve into a large bowl; discard solids. Season with salt and pepper. Chill soup for at least 3 hours or up to overnight. Garnish each serving with 1 teaspoon plain yogurt. Top with thinly sliced
cherry tomatoes if desired.