Clean mushrooms, cut large mushrooms in half. Clean and wash spring onions and cut them in diagonal pieces. Cut bacon into thin strips. Whisk eggs in a bowl, season well with salt, pepper and a little nutmeg.
Drain the bacon in a cast-iron, ovenproof pan (24 cm Ø) for about 3 minutes. Add the oil and mushrooms and fry over a high heat for 3-4 minutes while turning. Add the white of the spring onions and sauté for about 1 minute.
Add the eggs and allow to set at low heat for 8-10 minutes.
Switch on the oven grill (240 ° degrees). Cook the frittata under the grill on the middle shelf for 5-7 minutes. Carefully remove the frittata, spread the Bündnerfleisch on top and sprinkle with the remaining spring onions.
Mix yoghurt and mayonnaise and spread it with a spoon.