Place the potatoes in a large saucepan and pour in enough boiling water to cover them. Bring to the boil, then cook for 4 to 5 minutes or until they are just tender. Drain gently to ensure the slices do not break.
While the potatoes are cooking, heat the oil in a frying pan and brown the liver slices and diced lamb on all sides for about 1 minute. Transfer the liver to a casserole dish.
Add the bacon, onions, swede and carrots to the frying pan and cook, stirring, over a moderate heat for 6 minutes or until the bacon and onions are golden. Sprinkle the flour over the vegetables and stir it in, then stir in the stock. Add the rosemary, 2 teaspoons of the mustard and season to taste. Bring to a simmer and stir to thicken.
Pour the bacon and vegetable mixture over the lamb and liver in the casserole dish. Arrange the potato slices on top, overlapping them neatly and covering the vegetables, lamb and liver completely. Melt the butter in a small saucepan over a gentle heat and stir in the remaining mustard. Brush the mustard butter over the potatoes and season lightly.
Cook in the oven for 45 minutes or until the potatoes are browned and tender. Serve straight from the pot.