Forelle Blau, or "blue trout" gets its name from the trout's blueish hue when you douse the fish in hot vinegar.
It's very simple. First prepare a stock from onion, black pepper, salt, bay leaf, juniper berry and assorted green "soup" vegetables (I just had onion).
Boil the stock for 30 minutes.
In the meantime heat 250 ml of vinegar. Just plain vinegar or white wine vinegar.
When the vinegar boils, pour it over the trout. The trout can be washed, but the slimy outside layer should be still intact. It's the slime which turns blue. Lightly salt the trout on the inside.
Then slide the trout and vinegar in the stock and let it simmer for 15 minutes on a very low fire.
Serve with potatoes, green vegetables and Meerrettich Sahne (horseradish cream).