This is a traditional Northumberland dish which is also made in Ireland, sometimes called "farmhouse bake." Whatever its origins, it's just gotta be yummy with these ingredients!
Recipe: The Country Cooking of Ireland
Melt half the butter or bacon fat in a large oven-proof skillet, over low heat. Fry the bacon, stirring frequently for 10 to 15 minutes, until crispy. Remove from skillet and drain on paper towels.
Cook the onions in the same skillet for 6 to 8 minutes, until soft but not brown. Remove and add to the bacon, mixing them together.
Remove pan from heat and carefully lay a thin layer of potatoes on the bottom of the pan so that they completely cover the surface. Season to taste with salt and pepper. Cover with a layer of onions and bacon, about half of them, then a layer of cheese, about a third of it.
Add a layer of half the remaining potatoes and season lightly. Cover with all the remaining onions and bacon, then a layer of half the remaining cheese. Finish with a layer of the remaining potatoes and again season lightly.
Drizzle the remaining butter or bacon fat over the final layer of potatoes. Bake in a preheated 350-degree oven for 45 minutes.
Remove from oven and top with the remaining cheese. Place under broiler for 3 to 4 minutes until the cheese has melted and begun to brown. Serve immediately.