Place the potatoes in a large saucepan and pour in enough boiling water to cover them. Bring back to the boil, then cook for 4 to 5 minutes or until they are just tender. Drain well but 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 on both sides for about 1 minute, turning once. Transfer the liver to a casserole.
Add the pancetta, onions and mushrooms to the frying pan and cook, stirring, over a moderate heat for 10 minutes or until the pancetta is crisp and onions are golden. Sprinkle the flour and 2 teaspoons of the crushed coriander seeds over the vegetables and stir it in, then stir in the stock, lemon zest and lemon juice. Add the sage, parsley and oregano, and season to taste with salt and pepper. Bring to the boil, stirring.
Pour the pancetta and vegetable mixture over the liver. Arrange the potato slices on top, overlapping them neatly and covering the vegetables and liver completely. Brush the melted butter over the potatoes and sprinkle with the remaining 1 teaspoon coriander.
Bake in the preheated oven until the potatoes are browned and tender, about 45 minutes. Serve straight from the pot.
You can substitute streaky bacon for the pancetta. You can also use 1/2 teaspoon ground sage instead of fresh sage, in a pinch.