Peel the onion and saute in the oil and butter over a medium heat in a cast iron skillet.
Add the crushed or chopped garlic until both are soft and lightly browned - then drain and remove to a holding dish.
Peel and chop the beetroots into small cubes and place in skillet to soften in the butter/oil mix. Turning them now and again to avoid burning.
Add the stock and cover the pan while reducing the heast to a low, rolling simmer for 30-35 mins. You can test when its ready by slicing a beet cube - it should feel soft to cut.
Allow to cool then reduce with a blender to a smooth but thick liquid.
Season to taste and serve warm (but not hot) garnishing with a drizzle of plain greek yoghurt.