Put the beetroot into a large saucepan with the stock, caraway seeds, bay leaves, and bones (if you're using them).
Cook the beetroot for 1-1 1/2 hours until a knife enters with little resistance. Then, lift them out and run them under cold water for a couple of minutes to cool.
Set the stock aside, chucking out the bones. Give the pan a quick rinse then put it back over a medium heat.
Sweat the onions and cabbage in the butter, keeping the lid on the pan and being careful not to let them colour. Once the cabbage has started to soften (after 10-15 minutes), pour in the stock and bring to a slow simmer.
When the beets are cool enough to handle, peel and slice them into very rough matchsticks.
Add them to the pan, and once it's boiled, turn down the heat, add the tortellini and simmer for a couple more minutes.
Turn the heat off, adjust the seasoning, then serve with a blob of sour cream and a scattering of chives.