Preheat oven to 200°C/400°F/Gas Mark 6.

Cut the tomatoes in half and place them on a baking sheet, cut side up.

Roast the tomatoes in the oven for 20 minutes.

Melt the butter in a large pot over medium heat. Add the onion and garlic and cook until the onion is translucent, about 5 minutes.

Add the roasted tomatoes and vegetable stock to the pot. Bring to a simmer and cook for 15 minutes.

Remove the pot from the heat and use an immersion blender to puree the soup until smooth.

Stir in the sugar, salt, black pepper, and heavy cream. Cook over medium heat until heated through, about 5 minutes.

Serve with basil on top.