Jeni, Jammy's a man; up here men don't cook. As someone else said about Highland men - they just grow facial hair and do manly things with outboard motors, gruffly.
First, as in all things, whether cooking-related or not, you fry an onion in beef dripping. Or, if you're a big softy, a dash of olive oil. Hurl in as many chunks of raw tattie as you can be bothered peeling and chunking. Fry. Or, if you will: saute. Bung in enough stock to half-fill the pot. Bring to the boil, add illegal amounts of salt and pepper, hurl chunks of leftover cold roast beef into the mix along with leftover gravy and then put in a low oven for an hour or so. I use the bottom oven of my Aga but then I am poshish.
Serve when it's all disintegrated into thick mashy gloop. Optional accompaniments include HP sauce, baked beans, and a great big fish farmer.