1.Preheat oven to 180°C (160°C fan-forced).
2.Separate garlic bulbs into cloves. In a single layer, place unpeeled cloves on an oven tray. Drizzle with half of the oil. Roast, uncovered, 15 minutes or until garlic is soft. When cool enough to handle, squeeze garlic into small bowl and discard skins.
3.Heat remaining oil in large saucepan. Stir onion and thyme over heat until onion softens. Add potato and stock; bring to a boil. Reduce heat and simmer, uncovered, about 15 minutes or until potato is just tender. Stir in garlic; simmer, uncovered, 5 minutes.
4.In batches, blend or process soup until smooth. Return to same cleaned pan. Stir until hot then add cream. Divide soup among serving bowls. Serve sprinkled with extra thyme, if desired.