Wash and dry the fish, put on a pan covered with parchment.
Finely chop the onion and fry until soft in butter, add the garlic and zest from the lemon, passed through the press, for about 1 tsp., Let it cook for 1 minute, then add the juice from the lemon. Cook another minute.
Season the fish with salt and pepper to taste and put the onion dash evenly throughout the fillet.
Cook baked trout at 220 degrees 15 minutes, sprinkle with dill before serving and sprinkle with lemon juice.