Wash (and optionally peel) the potatoes and put in a pan of boiling water.
Chop the onions into thin slices and fry gently in a large frying pan with some of the oil or butter.
While the onions are frying prepare your green vegetables and further slice the liver if necessary.
Prepare a plate or wide bowl, covering it in flour mixed with a bit of salt and pepper. This is for dusting the liver prior to frying - easiest done if you arrange the liver, flour and frying pan in a straight line across your work-surface.
Once the potatoes are cooked - drain and then mash them, adding butter and cream or milk until you have the desired consistency. Put a lid on the pan to keep warm.
Once the onions are soft, remove them from the pan with a slotted spoon, squeezing the oil or butter back into the frying pan as you remove them. Put them in a bowl on one side.
Turn the heat of your frying pan up as high as possible and add a bit more oil or butter if necessary.
Start the process of dusting the slices of liver with flour on both sides and adding them to the hot pan. This needs to be done as quickly as possible and can be quite messy. Do not over-crowd the pan - if you find the liver takes up more surface-space than is available then leave the remains on one side and fry in a second batch.
The liver should be cooked fast, for only about a minute on each side so the slices are still pink within. Turn them over a couple of times. Quite a bit of the flour and blood will stick to the bottom of the pan. This is ok - don't scrape it off as it will be used for the gravy in a moment.
Once the liver is done to your satisfaction remove it from the pan and put directly onto warmed plates. Have someone else serve out the mash and vegetables while you finish the gravy..... The trick is to try and not let everything else get cold while you are doing this.
Leaving the pan on a high heat, put the onions back in, add a good splash of Hendersons, and stir vigorously while adding boiling water (even better, you can re-use the hot water from the vegetables - this can be a smooth manoeuvre, draining it into the gravy as you pass the pan over to be served onto plates), dissolving the juices stuck to the bottom of the pan. The gravy should thicken from the flour already stuck to the bottom of the pan. If it doesn't get thick enough you can add a bit more flour or cornflour (mixed with a little cold water in a small cup to avoid lumps). Once the gravy is hot and thickened, spoon generously over the plates.