In a large casserole, alternate layers of potatoes meat and onions. Sprinkle each layer with parsley, salt, pepper and thyme. As the last layer, put potato slices, pour water.
Bring to a boil and simmer on low heat for 2 hours with the lid closed. Add water if necessary.
Spread butter evenly on top and gratinate in oven until golden brown.
Serve with vegetables tossed in butter and Guiness.