Don't throw away leftover cooked - boiled cabbage or Brussel Sprout leaves - keep leftover mash potato as well - store in Food Bags that are suitable for the Fridge (or Freezer),and Plastic Containers with lids.
Onions are not ! used - but cutting streaky bacon into thin little bits with a pair of scissors and frying them in a generous amount of Olive Oil at the beginning - before adding the cabbage and then the mashed potatoes - can make the Bubble and Squeak a more interesting Recipe - if you over cook the thin bacon - then the taste of your Bubble and Squeak will have been affected.Cooked cold cabbage is what is mainly used with cold prepared mashed potatoes.