Preheat oven to 400 degrees

Butter bottom and sides of 7 x 11 inch baking dish

Boil potatoes for ~20 minutes, until tender. Drain well. Return to saucepen over very low heat and shake pot to remove moisture.

Slice onion halves into 4-5 segments. Melt 1/2 the butter in a heavy skillet and cook onions for ~ 20 minutes, or until softened.

Add remaining butter to potaatoes in pot and mash thoroughly. Mash in milk. Using a wooden spoon, stir potatoes briskly to incorporate a little air.

Spoon 1/2 potatoes into baking dish, smooth surface.

Layer cooked onions, sprinkle with pepper and sprinkle stilton over top.

Spoon in rest of potatoes and smooth with spoon. Sprinkle with parmesan

Bake 30 min until top is golden and edges are bubbling. Let stand for 5 minutes,