Peel the garlic and crush or finely chop it. Peel the onion and finely chop it. Trim the tops off the beetroot and give the beetroot a scrub. Coarsely grate through a box grater. Pick the parsley leaves off their stalks and set aside. Finely chop the parsley stalks.
Pour 1 tbsp oil into a pan and bring to a medium heat. Slide in the chopped onion, garlic and parsley stalks along with a little salt and pepper. Fry for 4-5 mins, stirring frequently.
Tip in the grated beetroot and the dried red lentils. Crumble in the stock cube. Cover with 800ml boiling water and stir well. Return to the boil, then cover the pan and simmer for 20 mins.
Use a stick blender to whizz the soup till smooth. No stick blender? Simply transfer the soup to a blender (in batches if necessary) and blitz. Pour the soup back into the pan to keep warm.
Stir 1 tbsp of the horseradish mustard through the soup. Taste and add more salt, pepper or horseradish mustard if you think it needs it. Ladle into bowls. Finely chop the parsley leaves and scatter them over the soup to serve.