Preheat your oven to 200°C/Fan 180°C/Gas 6. Scrub and peel the parsnips and chop them into small pieces. Scatter into a roasting tin and toss with 1 tbsp oil and a pinch of salt and pepper. Slide into the oven and roast for 20-25 mins, till tender.
Meanwhile, peel and finely chop the onion. Peel and crush or finely chop the garlic. Pick the thyme leaves off their sprigs. Peel the apple and slice into quarters. Cut out the core, and chop the apple into small chunks. Scrub the potatoes and dice them.
Pour 1 tbsp oil into a pan and warm to a medium heat. Slide in the chopped onion, garlic and thyme, and sprinkle in a little salt and pepper. Cook for 3-4 mins, stirring occasionally, till slightly softened.
Stir the potatoes and apple into the onions. Crumble in the stock cube and pour in 800ml boiling water. Return to the boil. Pop a lid on the pan and reduce to a simmer, then cook for 15-20 mins, till the potatoes are tender.
When the parsnips are cooked, add them to the soup. Use a stick blender to blitz till smooth. No stick blender? Ladle the soup into a blender (in batches if necessary), and blitz. Have a taste and add more salt and pepper if needed, then ladle the soup into warm bowls and serve.