Peel and finely chop the red onion. Trim and finely slice the celery. Peel and dice the carrot and potato. Warm a large pan on a medium-low heat for 1 min, then add 1 tbsp olive oil and the veg. Season, pop on a lid and fry, stirring once or twice, for 8 mins till starting to soften. You can cover the pan with a baking tray if you don’t have a lid.
While the veg fry, peel and coarsely grate the beetroot (see our tip, above). Quarter the apple, slice out the core, then coarsely grate it down to the skin. Stir the beetroot and apple into the veg. Pop the lid back on and fry together for 5 mins, stirring occasionally.
Fill your kettle and boil it. Crumble the stock cube into a heatproof jug. Pour in 800ml boiling water and stir. Pour the hot stock into the pan. Cover and simmer for 10-15 mins till the veg are tender.
While the veg simmer, spoon the yogurt into a bowl. Add ½ tbsp horseradish mustard and stir to mix. Taste and add more mustard if it needs it, and a crack of black pepper. Finely chop the chives.
Taste the soup and adjust the seasoning if needed. Ladle into warm bowls and top with the yogurt and chives to serve.