Clean the mushrooms, rinse briefly if necessary and drain well. Cut large mushrooms in half. Cut bacon into thin sticks. Whisk the eggs and spice with salt, pepper and a little nutmeg. Clean and wash spring onions, then cut them diagonally into rings.
Heat the oil in a cast-iron, ovenproof pan. Lightly fry the bacon in it. Add the mushrooms and fry over a high heat for 3-4 minutes while turning. Preheat oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see
manufacturer). Steam spring onions for approx. 1 minute, up to approx. 2 tablespoons. Add the eggs and let them simmer at low heat for 8-10 minutes.
Finish cooking the frittata in a hot oven for 10-15 minutes. In the meantime, stir the yoghurt and mayonnaise until smooth, season with salt and pepper. Remove the frittata. Spread Bündner meat on it and sprinkle with the remaining spring onions.
Spread yoghurt cream in strips with a spoon.