The day before, roast the juniper berries, fennel seeds and mustard seeds in a pan without fat.
Finely grind the spice mixture in a mortar. Wash the trout fillet and pat dry. Mix the salt, sugar, spice mixture, dill and lemon and orange peel in a small bowl.
Brush the trout fillet with it on all sides, wrap it in cling film and let it steep in the refrigerator overnight.
Before serving, wash the marinade off the fish.
Cut the trout fillet off the skin in strips and serve.