Put oven rack in middle position and preheat to 350°F.
Arrange tomatoes, cut sides up, in a single layer in a large shallow baking pan and add garlic to pan.
Drizzle tomatoes with oil and sprinkle with salt and pepper.
Roast tomatoes and garlic 1 hour, then cool in pan on a rack.
Cook onion, oregano, and sugar in butter in a 6- to 8-quart heavy pot over moderately low heat, stirring frequently, until onion is softened, about 5 minutes.
Add tomatoes, garlic, and stock and simmer, covered, 20 minutes.
Purée soup in batches in a blender (be careful with this hot liquid), then force through a sieve into cleaned pot, discarding the solids. If you blend it VERY well, its really not necessary to strain (I don't!).
Stir in cream and salt and pepper to taste and simmer 2 minutes.
**Soup can be made 1 day ahead and cooled, uncovered, then chilled, covered.