Boil the potatoes until soft, then mash the boiled potatoes, while stirring in butter and milk, 1/4 tsp salt and nutmeg.
While the potatoes are cooking, melt 4 tablespoons of the butter in a large pan on medium heat, add the chopped onions, minced beef carrots and cook for 5 minutes. Then add broth, Worcestershire sauce, salt and pepper and cook for another 5 minutes.
Preheat oven to 175 ° Celsius. Layer the meat mixture on the bottom of a casserole dish, add the mashed potatoes on top in even layers. Sprinkle grated cheese evenly on top of the mashed potatoes. Place the casserole dish in the oven and bake for about 20 minutes, until golden brown on top.