Gently fry the red onions with the maple syrup over a low to medium heat. Remove the frying pan from the hot plate once the onions are soft and have a marmalade-like consistency. Add salt and pepper to taste.
Chop the raw trout into small pieces. Mix in the shallots, tarragon, a drop of maple syrup, salt and pepper. Form the chopped trout into round ½ lb. (200 g) burger-like shapes. Fry the burgers in butter in a medium hot frying pan for around 3 minutes on each side.
Mix the rutabaga and fennel with the sour cream, apple cider vinegar, rapeseed oil, salt, and pepper.
Serve the burger on a hamburger bun, preferably home-made.