Heat oil in a large saucepan on medium to high heat and sauté onions until light brown.
Add the mince and cook until no longer pink, about five to six minutes.
Stir in the flour, add seasoning, sauces, the water that has been mixed with the beef stock and dried thyme. Mix well.
Reduce the heat and simmer for about 45 minutes. The meat must be tender.
Transfer the meat mixture to an oven dish.
Preheat oven to 180°C. Place potatoes in a large pot and cover with cold water. Cover pot and bring to a boil. Lower heat; simmer until tender, 20 to 25 minutes. Drain potatoes and mash them.
Combine nutmeg, cream and egg yolk with butter, salt, and pepper. Mix into mashed potatoes.
Add the potato mash on the mince, spread evenly with a fork. Sprinkle with the cheese.