Measure out 2 tsp of the caraway seeds. Add them to a dry pan and toast over a medium heat for 2 mins till the seeds start to pop and smell nutty. Tip into a bowl.
Peel and finely chop the onion and potato. Put the pan back on the heat. Add 1 tbsp olive oil and the chopped veg. Season with salt and pepper. Cover. Keep the heat low and sweat the veg for 10 mins till soft.
While the veg sweat, slice the core out of the cabbage half. Roughly chop the leaves. Stir half the caraway seeds into the pan. Add half the dill sprigs. Crumble in the stock cube, then pour in 800ml boiling water (or use hot homemade stock). Cover, turn the heat up and bring to the boil. Simmer for 5 mins.
Add the cabbage. Simmer, without the lid on, for 5 mins to cook the cabbage. Ladle the soup into a blender and blitz till smooth, or use a hand-held blender in the pan. Taste and adjust the seasoning. Serve in warm bowls, garnished with a few dill leaves and the rest of the caraway seeds.