Preheat the oven to 200°C/gas mark 6. Put the beetroot in a large bowl and toss with 1 tbsp of the oil, the cumin seeds, thyme and a good grinding of black pepper. Tip into a roasting tin and roast in the oven for 30 minutes until the beetroot is really tender when pierced with a knife.
Meanwhile, heat the butter and remaining oil in a saucepan. Add the onion and cook for 5 minutes until softened. Add the chopped apple and cook for a further few minutes.
Add the roasted beetroot to the pan with the stock and cook for 10 minutes. Using a stick blender or jug blender, whizz the soup until smooth. To serve, add a spoonful of yogurt and finish with a sprinkling of seeds.