Place the tomatoes and peppers onto a baking sheet and sprinkle with the coriander seeds, cumin seeds, salt and pepper, and drizzle with olive oil. Bake until the tomatoes are very soft and the peppers are starting to caramelize, about 45 minutes. Scrape into a medium saucepan, using a rubber spatula to remove all the juices, and then add the vegetable broth. Puree with an immersion blender, and then taste for seasoning, adding salt and pepper if needed. Ladle the warm soup into bowls and garnish with a sprinkling of cilantro. Serve warm or cold.