Traditionally, Lancashire hotpot would be made using tough old mutton, hence the long cooking time. Mutton is hard to find these days, so you can use lamb, cutlets, neck or shin, on the bone if possible.
Preheat your oven to 170°C (338°F). If your meat is boneless, cut it into bite-sized chunks. Dust it with flour and sauté in oil to brown the meat all over.
Peel and slice the potatoes and onions. The slices should be 4-5mm (quarter of an inch) thick.
Grease a tall ovenproof dish with butter and arrange one third of the potatoes in a single layer on the bottom. Layer the meat over the potatoes and then arrange the onion on top of that. Pour in your stock until it just covers the onions. Arrange the remaining potatoes in a single layer, overlapping slightly. Melt a generous knob of butter and pour it all over the potatoes.
I should point out this is a bit of a peasant dish - people migh not have stock available and would just use water - this works perfectly well. Some recipes add thyme, bay leaves, etc. This is fine, but again probably not authentic.
Put the pot in the oven and bake for 2 hours. Check that the potatoes are cooked and serve with pickled red cabbage.