Saw this one recently in a British tabloid. It's supposedly from the chef at Buckingham Palace, so if it's good enough for Kate and Wil, it's fine by me. I cooked it exactly as written and it turned out great.
In a large saucepan, over high heat, saute the chopped onions until soft and translucent. Add the ground beef and break up using a wooden spoon. Saute until it is browned. Gradually stir in the flour, thyme, water, bouilon cubes and worcestershire sauce.
Reduce heat to low and simmer the meat(so it is just bubbling) for about 30 minutes. Remove from stove. Spoon onto a large pie dish. Leave to cool.
Place the potatoes in a large pot and cover with cold water(about an inch above the potatoes). Cover and bring to boil over high heat. Lower heat to a simmer until the potatoes are tender(about 20 minutes). Drain the water off the potatoes and return dry potatoes to a low heat for one or two minutes to dry them. Mash the potatoes with a potatoe masher.
Add the butter, salt, pepper, nutmeg, egg yolk and cream and stir into the potatoe mix. Spoon on top of the beef mix and sprinkle cheese over the potatoes.
Bake in a pre-heated oven (350 degrees) for about 20-30 minutes until the potatoes are golden brown and pie has heated through.