In a separate saucepan, place beetroots, carrots, celery, celery root, bay leaf, allspice, peppercorns and stock cube, cover with water and bring to a boil. Simmer until beetroots are tender. Season with salt and pepper.
Add vinegar and simmer until the soup is a bright red colour. Remove from heat.
In a frypan, melt 2 tablespoons butter and fry half the diced onion 5 minutes. Add to soup.
Mash the potatoes with remaining butter and place in bowls. Top with soup and garnish with remaining onion as desired.