Preheat oven to 400°F (200°C).

On a large rimmed baking sheet, arrange halved tomatoes, quartered onion, and the whole garlic head with the cut-side facing down.

Drizzle all vegetables with olive oil and season evenly with salt and pepper. Roast for 45 to 50 minutes until softened and slightly charred.

Carefully squeeze the roasted garlic cloves out of their skins and into a large heavy-bottomed pot or Dutch oven.

Transfer the roasted tomatoes, onions, and all collected juices from the baking sheet into the pot.

Add the broth and bring the mixture to a gentle simmer over medium heat for 10 minutes to meld flavors.

Incorporate the fresh basil leaves into the pot.

Use an immersion blender to process the soup until a completely smooth and velvety consistency is achieved.

Stir in the heavy cream and sugar, heating through over low heat for 3 minutes without reaching a boil.

Season with additional salt and pepper to taste before serving with grilled cheese or crusty bread.