Preheat oven to 200°C.
Peel eggs und halve them lengthwise. Remove yellow of the eggs and chop them. Heat butter and braise onion lightly. Add mushrooms, tomatoes and parsley and stew all for some minutes. Stir in yolk and leave to cool down. Season to taste with salt and pepper. Cover a gratin dish with butter.
Fill halves of eggs and place them into the gratin dish. Gratinate in oven about 15 - 20 minutes until golden brown on the top.