Roughly chop the cheese, remove the skin from the sausage and finely chop the spring onion. Wash and dry the potatoes then, using an apple coring tool (available in any cook shop) take out a core lengthways from each potato, saving the cores.
Using your fingers, push a mixture of butter, cheese, sausage and onion into each potato. Cut the potato cores in two and use one half at each end to block the hole so that the mix is totally contained within the potato. Wrap each potato with bacon rashers (slices) and then wrap in aluminium foil.
Bake in the centre of a preheated moderately hot oven (Mk 5 - 375ºF - 190ºC) for 1¼ - 1¾ hours until the potatoes are cooked. The cooking time will depend on the size of your potatoes, smaller potatoes being cooked in about 1¼ hours while bigger potatoes will take longer.
Once you are used to cooking roast potatoes, you will know just by giving them a gentle squeeze when they are done (they should give way slightly when squeezed between finger and thumb). If you are not used to it, insert a fork into one of the potatoes; it should go in easily without being forced. Remove the foil, cut the potatoes in two, lengthways, and serve hot with sour cream.