Preheat oven to 425°F (218°C). Arrange tomatoes, onion and garlic on a rimmed baking sheet.
Drizzle with olive oil, season with kosher salt and pepper, then roast for 30 minutes until edges turn golden.
Blend and Simmer
Let the vegetables rest for 5 minutes, then transfer to a blender. Add vegetable broth and basil leaves, and blend until silky smooth.
Pour the puree into a pot, bring to a gentle simmer over medium heat, and cook for 5 minutes.
Stir in heavy cream and sugar if using, and heat for 2 more minutes.
Adjust salt and pepper to taste, then serve garnished with fresh basil leaves.
Notes
Store in an airtight container in the refrigerator for up to 4 days.Freeze in freezer-safe containers for up to 3 months; thaw overnight before reheating.