Heat your oven to 200°C/Fan 180°C/Gas 6. Set a roasting tin on the top shelf to warm up.
Peel or scrub the carrots. Cut into 2-3cm chunks. Peel and core the apples and cut them into chunks a little bigger than the carrots. Tumble the carrots and apples into the warm roasting tin. Gloss with 1 tbsp oil and season well with salt and pepper. Roast for 30 mins, or till golden and tender. Pop the garlic cloves (whole and in their skins) into the roasting tin halfway through the cooking time.
While the veg roast, peel and finely chop the onion. Finely dice the celery. Warm a large pan for 1 min. Add 1 tbsp oil. Swirl in the onion and celery with a pinch of salt and pepper. Sizzle over a low heat for 10 mins till tender. Crumble in the stock cube. Pour in 700ml boiling water. Gently simmer while the rest of the veg roast.
When the roast veg are ready, pluck the garlic cloves from the tin and strip them of their papery skins. Add the roast veg and garlic to the simmering stock. Cook for 5 mins more, to bring the flavours together.
Add most of the thyme leaves to the soup. Blend till smooth, either by ladling into your blender or using a hand-held blender in the pan. Taste and adjust the seasoning as needed. Serve garnished with the remaining thyme leaves.