In the oil and a little broth, sauté the chopped onion, add the minced meat and continue sautéing until the water evaporates. Add salt, spices and pour on the hot broth (or water). Boil in a covered container.
Meanwhile, mash the boiled potatoes, add the milk and a little salt. Put half of the mashed potatoes in a buttered baking dish, throw on the minced meat, cover with the remaining mashed potatoes.
Arrange onion slices, pieces of butter and bake in a hot oven. Beat the ingredients for the topping together. Pour it over the baked moussaka and then return to the oven until the topping crisps.