Boerenkool met worst

This is a typical Dutch dish, "boerenkool met worst", translated something like farmers cabbage with sausage.
recipe
1.5 kg potatoes, was very well
250ml water
1 bag of "boerenkool"
salt
1 rookworst (special kind of sausage in Holland)
1 dl milk
40 gr. butter
Clean the potatoes, cut each potatoe in 4 parts and wash them very well.
Take a very big pan, put the water in together with the potatoes.
Add the salt (I like it with a lot of salt). On top of that you put the boerenkool.
On top of the boerenkool you put the sausage. Wash it fist under hot water.
Cook for 30 minutes (low fire)
After that, use a "stamper" to mix everything, add the butter and the hot milk.
I like it together with mayonaise, but not all people like that.