Preheat the oven to 400°F (200°C).
Place the halved tomatoes, quartered onion, and garlic cloves on a large baking sheet.
Drizzle the olive oil over the vegetables and sprinkle with kosher salt, black pepper, and dried oregano. Toss gently to coat evenly.
Roast the vegetables in the preheated oven for 50 minutes, or until the tomatoes are soft and slightly caramelized.
Remove the baking sheet from the oven and let the vegetables cool slightly.
Transfer the roasted tomatoes, onion, and garlic to a large pot.
Add the vegetable broth, tomato paste, and sugar to the pot. Stir to combine.
Bring the mixture to a simmer over medium heat and cook for 10 minutes to meld the flavors.
Using an immersion blender, blend the soup until smooth and creamy. Alternatively, carefully transfer the soup in batches to a blender and puree until smooth, then return to the pot.
Stir in the heavy cream and chopped fresh basil leaves. Heat the soup gently for 2-3 minutes without boiling.
Taste and adjust seasoning with additional salt and pepper if needed.
Ladle the creamy roasted tomato soup into bowls and serve warm.