Preheat oven to 400°F (200°C). Place halved tomatoes, onion quarters, and the head of garlic (cut side up) on a baking sheet. Drizzle generously with olive oil, salt, and pepper.
Roast for 40 minutes, or until the tomatoes are blistered and collapsing, and the garlic is soft and caramelized.
Transfer the roasted tomatoes, onions, and any juices from the pan into a large pot. Squeeze the roasted garlic cloves out of their skins into the pot.
Add the vegetable broth and fresh basil leaves. Bring to a gentle simmer.
Use an immersion blender to puree the soup until smooth. Stir in the heavy cream and let it warm through (do not boil). Serve hot!