Heat your oven to 200°C/Fan 180°C/Gas 6. Set a roasting tin on the top shelf of your oven to heat up.
Peel or scrub your carrots. Cut into 2-3cm chunks. Peel and core your apples. Cut into chunks a little bigger than your carrots. Tumble into the warmed roasting tray. Gloss with 1 tbsp oil. 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 cooking.
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 low heat till tender. Crumble in the stock cube. Pour in 700ml boiling water. Let it simmer away while your vegetables roast.
Once the roast veg are ready, pluck the garlic cloves from the tin. Strip them of their papery skin. Add the roast veg and garlic to your simmering stock. Simmer for 5 mins longer to marry the flavours.
Add a small handful of fresh mint leaves to the soup. Blend till smooth. Taste. Season as needed. Add more mint if you like. Serve garnished with the remaining mint leaves.