Which British dish is a stew of lamb, onions, and potatoes traditionally cooked in a heavy pot?

The British stew of lamb, onions, and potatoes traditionally cooked in a heavy pot is Lancashire hotpot.

The dish usually consists of lamb or mutton layered with onions and thinly sliced potatoes. It is cooked slowly in a covered or lidded pot until the meat is tender and the potato topping becomes browned. Oysters were included in some historic Lancashire recipes, though they are not essential to modern versions.

Lancashire hotpot is associated with the county of Lancashire in northwest England and with the region’s industrial communities. Its slow cooking made it practical for households that needed a filling meal from inexpensive ingredients.

Unlike shepherd’s pie, Lancashire hotpot does not traditionally use mashed potato as a separate topping. The characteristic potato slices form a layered crust. The dish is also different from Irish stew, whose customary ingredients and regional identity belong to Ireland.