Salt can be reduced, as the gravy will balance out.
Adding vinegar to the onions and reducing them gives the sauce a nice sharp edge; this also helps to reduce the amount of salt needed to make a nice sauce.
All butter can be removed from the mash, as you have plenty of gravy to soften the mash.
Great, simple food which can be made the day before and re-heated.
If you do not have granules, add 1 tbsp of flour to the cooked and reduced onions, then add water and a beef stock cube, this will thicken perfectly.