Drain the cooked potatoes. In the saucepan you used to cook the potatoes, heat the remaining 3/4 cup milk and the remaining 2 tablespoons butter until barely simmering. Put the cooked potatoes through a food mill, a ricer, or use a potato masher to mash and add them to the simmering milk. Stir in the cheddar
, season with salt and pepper and stir to blend well, until the cheese is melted and incorporated. Spread the mashed potatoes evenly over the lamb mixture in the pie pan. Use a spoon to make nice wavy peaks and valleys in the potatoes; the peaks will then get nicely browned and beautiful in the baking.