Chop the onion and fry it briefly with the minced meat in preheated oil. Sprinkle it with flour, season it with salt, pepper and oregano. Add peas and chopped carrots, pour in some hot water and let it stew.
Boil the potatoes, mash them and mix with the butter. Once slightly cooled, mix with the egg, that has been whisked in advance. Put the mixture with minced meat and vegetables in a greased pan and spread the top nicely with mashed potatoes.
Put it to bake in a warm oven for 30 minutes at 392°F (200 °C). Sprinkle it with grated cheese, if desired.