The above rather lengthy list of ingredients comes from Chef John, but the basic principle is simple.
Fry the ground lamb and onion together and keep at it until the water cooks off. It will take about 10 minutes–don't rush this step.
Add garlic and flour and cook until the flour is absorbed. Then add dry spices and do the same.
Add water and catsup. Cook until everything gets nice and thick. This step will also take a while. The result is a nice thick flavorful gravy.
Meanwhile, cook and mash the potatoes. Add the butter, cream, cheese, salt and pepper. Beat an egg and milk together. By the time you've mashed the potatoes, they will have cooled a little. Beat in the egg.
Then layer the pie and bake it for 25-30 minutes until the mashed potatoes are brown and everything is hot and bubbly.
It may seem weird to cook shepherd's pie from scratch, but Uly and I rarely have the right kind of leftovers in the right quantities to make it from leftovers.