Pour the milk over the bread and let stand for an hour.
Heat the butter and steam the onion. Crush the bread with a whisk and mix in the cheese.
Pour the mixture into the pan with the onion, mix. Heat everything until the cheese has melted.
Serve with roasted onions.
Sarganserländer cheese soup is a typical Älpler (alpine farmer) dish from olden times, when people didn’t have much else on the alp