Preheat the oven to 400 degrees F. Drain the tomatoes, reserving the juice. Cut the tomatoes in half. Toss in a medium size bowl with onions and garlic and about 4 tablespoons of olive oil. Liberally sprinkle with salt and pepper.
Spread evenly on a baking sheet and roast until onions turn brown, about 30 minutes, stirring halfway through to prevent burning.
Transfer the tomato mixture to a large heavy bottomed pot. Add the reserved tomato juice, stock and fresh thyme leaves. Let simmer for about 15 minutes. Blend until smooth, taking care not to splatter the hot soup. Add heavy cream and return to a simmer, but do not boil. Adjust the seasoning.
Pour into bowls and top with a thin strips of fresh basil.