Two and a half hours at 160 °C, covered for the first two so the meat cooks in its own steam and the potatoes cook in the fat that comes off it. Do not add liquid: the lamb and the onions give up more than enough, and anything you pour in boils the dish instead of roasting it.
It did not make it into the plan for these seven days.
The method above is what decides the dish, not a step-by-step recipe. The link runs a web search on the name: this page has no vetted sources of its own, and will not invent any.