Set a saucepan over a medium heat and add the butter and olive oil.
When the butter is foaming, add onion, fry gently for 10 mins, then add the garlic and cumin and cook gently for 2-3 minutes.
Add the beetroot chunks, toss well with the onions, then add the stock.
Bring to a simmer, cover and cook for 20-25 minutes, until the beetroot is tender.
Put the cumin, coriander and caraway seeds in a dry pan and toast over medium heat till fragrant.
Grind the spices to a powder, then combine with paprika, cayenne and a pinch of salt.
Stir this mixture into the yogurt, add the olive oil and whisk together.
Purée the soup, return it to the pan and season to taste, check the thickness.
Serve the soup, hot or chilled, with a dollop of spiced yogurt swirled in at the last moment and some parsley sprinkled on top.