Preheat oven to 400 degrees. Line a baking sheet with foil.
Slice and "X" into the bottom of each tomato and place on the prepared baking sheet. Drizzle with olive oil. Sprinkle with thyme and rosemary. Toss tomatoes until well coated.
Roast in preheated oven for 30 minutes, then broil for another 15 to 20 minutes, until slightly charred on top. Keep a close eye on them at this point--it is easy to burn them!
While the tomatoes are roasting, heat the cooking oil in a soup pot over medium heat. Sauté the onion and garlic for 4-5 minutes, until onion is translucent. Season with salt.
Transfer the roasted tomatoes to the pot with the onion and garlic, and stir well. Add broth. Bring to a simmer. Add cheese, and stir until it melts.
For a smooth soup, transfer to a blender and puree until smooth. For a chunky soup, you can use an immersion blender right in the pot.
Divide soup between two serving bowls. Top with a dollop of crème fraîche and fresh basil and sorrel (if using).