Potato-wise, leftover cold potatoes are ideal, but you can boil uncooked ones if you are starting from scratch. My preference is for floury potatoes, as they crumble in the pan, soaking up more fat to give a crunchier finish. New potatoes cook more neatly and evenly, but have less charm.
I added some thyme leaves, as I had them in my fridge, but they are a mere suggestion, not an instruction. The tougher herbs (rosemary, thyme, sage) would all go well if you have them, but don't rush out to buy extras, thus starting an endless cycle of leftover ingredients. For similar reasons, I haven't specified exact quantities. Use what you have to hand. Obviously you don't want a dish that is all mostly just meat or just potato, just go with what looks right and what you feel like eating.
A fried egg is not actually part of the hash itself, but for me it completes its, as does ketchup and a good shake of Tabasco.
Oil, dripping or lard
Onion (I reckon 1 large onion for 4 people is about right)
Cooked meat (in this case, salt beef, but you can also use leftovers from a roast or corned beef)
1 or 2 eggs per person (if you like)
salt and pepper.
Heat your chosen fat in a large, heavy based frying pan. Dice up your potato into a size that pleases you, and once the oil is hot, add the potato pieces to the pan. Even out over the pan so they get browned on one side. Meanwhile, chop the onion and meat. Turn the potatoes over when they are nicely coloured, making sure to scrape the browned bits off the base of the pan. When all the potatoes have a golden colour, add the onions and cook for a few minutes until they start to soften. Add the meat and allow to crisp slightly- you may need to turn the heat up towards the end. If you are having eggs, push the potatoey mix to one side of the pan and fry them alongside. Done.