On a large baking sheet, arrange the halved tomatoes, chopped onion, and unpeeled garlic cloves. Drizzle with olive oil and season with salt, black pepper, and sugar.
Roast in the preheated oven for approximately 30-35 minutes, turning the tomatoes halfway through.
Allow the garlic to cool, then squeeze the soft roasted garlic from their skins into a large pot.
Transfer the roasted tomatoes and chopped onion into the pot with the garlic. Pour in the vegetable broth and bring to a gentle simmer over medium heat.
Simmer for about 10 minutes, then blend the soup with an immersion blender to your desired smoothness.
Add the fresh basil leaves and blend again until fully incorporated.
If desired, stir in the heavy cream and gently heat through without boiling.
Taste and adjust seasoning as necessary.
Notes
Serve hot with a drizzle of olive oil and fresh basil leaves. Pairs well with crusty bread or grilled cheese.