Preheat your oven to 400°F (200°C). Line a large baking sheet with parchment paper.
Place the tomatoes, peppers, onion, and garlic on the baking sheet. Drizzle with olive oil and season with salt and pepper. Toss everything to coat evenly.
Roast the vegetables for 35-40 minutes, or until they're softened and slightly charred around the edges. The tomatoes should be bursting and juicy.
Transfer the roasted vegetables to a large pot. Add the vegetable broth, balsamic vinegar, and thyme.
Using an immersion blender, blend until smooth. If you don't have an immersion blender, carefully transfer the mixture to a regular blender in batches.
Simmer the soup over medium heat for 10-15 minutes to let the flavors meld together.
Stir in the heavy cream if using, and adjust seasoning to taste.