3/4 cup grated cheese (gruyère, emmental, or similar)
In a large frying pan, cook the onions and garlic in the butter and olive oil on medium heat for about 5 minutes. Stir in the tomatoes, ground beef, herbs, salt and pepper. Cook until the meat is browned thoroughly. Turn off heat and add egg yolk and Parmesan cheese, stirring to mix completely.
Spread the meat in the bottom of an lightly oiled oven proof dish (a 13 X 9 inch Pyrex dish would be perfect for the amount given in this recipe). Spread the potatoes on top of this and finish by sprinkling grated cheese on top.