Preheat your oven to 400°F (200°C). On a baking sheet, toss the halved tomatoes and chopped onion with 2 tablespoons of olive oil, salt, and pepper. Spread them out in a single layer. Roast in the oven for about 30-35 minutes until the tomatoes are soft, slightly caramelized, and the onions are golden.
While the vegetables are roasting, heat a large pot over medium heat. Add a splash of olive oil, then sauté the minced garlic until fragrant, about 1-2 minutes. Be careful not to burn it as garlic turns bitter quickly.
Carefully transfer the roasted tomatoes and onions to the pot with the garlic. Pour in 2 cups of vegetable broth. Bring the mixture to a gentle simmer and let it cook for 10 minutes to marry the flavors.
Stir in the chopped fresh basil leaves, salt, pepper, and if using, 1 tablespoon of sugar to balance the acidity. Simmer for an additional 5 minutes.
Using an immersion blender, blend the soup directly in the pot until smooth and creamy. Alternatively, carefully transfer the soup in batches to a countertop blender and puree until velvety.
If the soup is too thick, add a little more broth until you reach your desired consistency. For an extra touch of indulgence, swirl in a splash of heavy cream when serving. Ladle into bowls and garnish with fresh basil leaves or a drizzle of olive oil.