Peel, wash and cut the potatoes into pieces. Cook in boiling salted water for about 20 minutes. Clean, wash and cut spring onions into rings. Clean, wash and halve the mushrooms.
Wash the cherry tomatoes, dab dry and cut in half. Peel and slice the onion. Wash the meat, dab dry and cut into cubes. Heat oil in a large pan and fry meat all around.
Add prepared vegetables and fry for 2-3 minutes. Wash oregano and dab dry. Add tomato paste to the meat, sweat briefly and deglaze with strained tomatoes and tomato juice. Add oregano, except for something to garnish, and simmer for 2-3 minutes.
Season to taste with salt, pepper and sugar. Remove the oregano. 250 ml milk and warm up. Drain the potatoes and let them evaporate briefly. Add milk-fat mixture, add crème fraîche and mash everything to a puree.
Season to taste with salt and nutmeg. Grease 2 smaller or one large casserole dish. Pour the puree onto the rim. Put the casserole ragout in the middle. Whisk the egg yolks and carefully brush the puree with them. Gratinate under the preheated grill of the oven until golden brown.
Cover the end of the ragout if necessary. Remove and garnish with oregano.