reminded me of this simple but wonderful recipe:
Ingredients (for 3-4 people):
Peel the potatoes and cook in a large pot with lots of water and salt until they offer no resistance to a skewer going through the middle.
Chop the onion finely and fry until brown and almost crispy.
Chop the mushrooms and fry them until fairly dark and shrivelled.
Mash the spuds the butter until melted, then add about 1/3 of a cup of milk and mash that in as well. Season well at this stage.
Mash in the egg yolk - this will help with cohesion and add colour.
Finally, stir in the fried onions and 'shrooms and serve presto - best eaten piping hot!