Heat olive oil in a large saucepan over medium heat. Cook and stir onion and garlic in hot oil until fragrant, 2 to 3 minutes.

Pour vegetable broth over the onion mixture; add tomatoes, 1 tablespoon cilantro, sugar, and red pepper flakes. Bring the mixture to a boil, reduce heat to medium-low, place a cover on the saucepan, and simmer until the tomatoes are really soft, about 15 minutes.

Pour tomato mixture into a blender no more than half full. Cover and hold lid in place; pulse a few times before leaving on to blend. Puree in batches until smooth and return to the saucepan.

Put pureed soup over medium heat and cook until hot, about 5 minutes. Stir remaining tablespoon cilantro and the heavy cream into the soup.