After scrubbing the potatoes peel or leave skins on; cut potatoes in good size pieces and boil in salted water until tender. This can be done by poking a sharp small knife into a potato chunk, if it slides off the potatoes are ready to be strained and mashed.
Once strained return the potatoes to the saucepan, add the butter and some of the hot milk and begin to mash gradually adding all the milk. The consistancy of the potatoes should be thick and creamy if you should have any lumps remove them. Season with salt and black pepper.
Core the head of cabbage and cut into pieces, place into cold water with the scallions and bring to a boil. Cook until turns to a bit of a darker colour and the flesh has soften. Add a couple of tablespoons of butter and cover with a lid for a couple of minutes in order to tenderize the cabbage.
Drain the cabbage thoroughly then chop it into small pieces. Crumble the cooked bacon and combine the bacon, cabbage and scallion with the mashed potatoes, stirring gently together.
What I do with my 'bubble & squeak' is place the potato mixture into pan and decorate using a wooden spatula in a swirling motion and brown under the broiler until slightly browned. As an addition sprinkle some grated cheddar cheese over the top before putting in under the broiler, of course this is optional.