Put the peas in a pot with 1 litre of water, cover and let it boil soft for about 1 hour. Peel and wash the potatoes and cut them into pieces. Cook in boiling salted water for about 20 minutes.
Wash and clean the tomatoes and cut them into pieces. Peel onion and chop very finely. Wash the meat, dab dry. Heat oil in a pan, fry meat on each side for 3-5 minutes. Remove, season with salt and pepper and keep warm.
Sauté the onion and tomatoes in the frying fat. Add oregano. Add 125 ml water, stir in mustard, season with salt and pepper and simmer for 2-3 minutes. Drain the potatoes and let them evaporate briefly.
Add the milk and butter, mash to a puree. Drain the peas, drain and mix into the purée. Season to taste with salt and nutmeg. Season sauce to taste as well. Serve the puree with steaks and sauce.
Garnish with oregano.