Wash, peel and roughly chop the vegetables.
Heat the oil in a saucepan and gently cook the onion, garlic, celery and carrots for about 10 minutes, until soft.
Meanwhile, peel and dice the apple.
Add the beetroot and apple pieces to the pan, sauté for 5 minutes then add the stock. Season with salt and pepper and cook, under cover, for about 30-40 minutes until the beetroot is tender.
Liquidize the soup or, using a hand-held blender, process until smooth.
Serve hot with a dollop of sour cream and feta cheese crumbled on top if you like.