Preheat your oven to 200°C/Gas 6. Set a roasting tray on the top shelf to heat up.
Peel or scrub your carrots. Cut into 2-3cm chunks. Peel and core your apple. Cut into chunks a little bigger than your carrots. Tumble into the warmed roasting tray. Gloss with oil. Season well. Roast 30 mins or till golden and tender. Pop the garlic cloves (left whole and in their skins) half way through cooking.
Peel and finely chop your onion. Wash your celery and finely dice. Get a large pot hot. Add a gloss of oil. Swirl in the onion and celery with a pinch of salt and pepper. Sizzle over low heat till tender. Pour in 700ml water. Let it simmer away while your roasted veg cooks.
Once the roasted veg is done, pluck the garlic cloves from the tin. Strip them of their papery skin. Add all the roasted veg to your simmering stock. Simmer 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.