Peel the onions and cut in half length ways, then slice length ways again so each half is cut into 4 long pieces. Heat the oil in a casserole dish that will also go into the oven, and add the onion.
Cut each of the sausages in half and add to the casserole dish. Fry gently for about 5 minutes, making sure the onion doesn't burn, stirring every so often so that the sausages brown evenly.
Add the sugar and the balsamic vinegar and cook gently for another five minutes.
Meanwhile, peel the beetroot, cut each in half, then slice each length ways in the same manner as the onions. Add to the casserole dish along with the tinned tomatoes and some freshly ground black pepper and a little salt. Quarter fill the empty tomato tin with water and add to the pan.
Put the lid on and place in an oven pre-heated to 200 centigrade.
Meanwhile, prepare the potatoes - if they are small then they can stay whole, but if you are using large ones then dice them into small pieces.
Once the casserole has been in the oven for 15 minutes, add the potatoes, stir, and place back in the oven for another 30 - 40 minutes. If you need to delay dinner for any reason, the casserole will happily sit in the oven much longer if you turn the heat down. Check every so often to make sure the casserole isn't drying out - simply add a little more water if too much liquid has evaporated.
If serving with cabbage, then chop and put on to steam about 10 minutes before the end of cooking time.
Once ready to serve, check the potatoes and beetroot are cooked by sticking a sharp knife into them. Check the seasoning and adjust if necessary. Serve with the cabbage and with fresh bread to mop up any excess juices.