Wash the potatoes and cook in salted water for about 20 minutes. Defrost peas. Boil the egg for 8-10 minutes until soft. Quench and let it cool down. Wash and quarter mushrooms.
Chop onion finely. Leave ham without fat crispy. Add onion and mushrooms and stir-fry for about 5 minutes. Season with salt and pepper. Deglaze with 100 ml water, bring to the boil. Stir in stock and thicken. Stir in cream. Drain the potatoes, except for some potato water. Add milk and mash finely. Season to taste with salt, pepper and nutmeg.
Stir in cream. Drain the potatoes, except for some potato water. Add milk and mash finely. Season to taste with salt, pepper and nutmeg. Fold in peas and heat up. Peel the egg, cut in half. Arrange everything and sprinkle with cress if necessary