To make beef base: In a large saucepan over a high flame, heat the oil and add the finely chopped onion. Sauté until the onion becomes soft and translucent. Add the ground beef and break it up using a wooden spoon. Sauté until the beef has lost all its pink color. Gradually stir in the flour, thyme, 2 cups water, Kitchen Bouquet seasoning, beef bouillon cubes and Worcestershire sauce.
Reduce the heat to low and simmer the meat (so that it is just bubbling) for about 30 to 45 minutes until the sauce has thickened and the meat is fork tender. Remove the pan from the stove, and use a slotted spoon to transfer the beef to the earthenware dish. The slotted spoon will lift just the beef so it will not be too wet to hold the potatoes.
If there is any sauce left after taking out the meat, cool and refrigerate it. Then reheat it in a small saucepan to serve alongside the finished pie. This recipe can be prepared up to this stage several days before you plan to serve the dish. Cool both the meat and the liquid and store in the refrigerator.
To make potato topping: Place the potatoes in a large pot and add cold water to about 1 inch above the potatoes. Cover the pot and bring to a boil over high heat. Lower the heat to a simmer and cook until the potatoes are tender when pricked with a fork (about 20 minutes). Drain off the water and replace the drained potatoes in the pan over a low heat for 1 to 2 minutes to dry them thoroughly. Mash the potatoes with a potato masher or pass through a ricer.
Stir the nutmeg, cream and egg yolk with the butter, salt and pepper into the mashed potatoes. Then use the mashed potatoes to cover the top of the dish. Either pipe them, using a wide star tube, or spoon them on and then fluff up using a fork.
Sprinkle the top of the potatoes with the grated Cheddar. It can be kept for 24 hours at this stage if covered with plastic wrap.
Bake in a preheated oven (350°) for about 20 to 30 minutes until the potatoes are golden brown and the pie has heated through.