Boil potatoes till soft then use a masher and mash adding butter and cream. Season with salt and freshly ground pepper.
Assemble by placing the lamb at bottom layer of your pan and the mashed potatoes on top. Use a fork to draw some lines. Use as much mesh or lamb as you like but half half is good.
At this point you can wrap nicely and keep in your fridge or freezer or bake at 200C for 20 - 25 minutes to brown the top before serving. if you took out from freezer, size depending then set your oven to 180C and cook for at least 40 - 45 minutes, keeping an eye to judge by the brownness on top.