Rinse the fish under cold running water. Pat dry with paper towels.
In a small skillet, toast the coriander, cumin, dill, and fennel seeds over medium heat for about 2 minutes, until they start to release their fragrance. Transfer to a small bowl, add the salt, and mix well.
Rub the fish with the spice mixture and place in a baking dish. Sprinkle with 1 tablespoon of the lemon juice. Cover and let marinate in the refrigerator for 1 to 2 hours.
To prepare the aquavit sour cream, in a small bowl, mix together the sour cream, aquavit, caraway seeds, and chervil. Add vinegar to taste. Cover and refrigerate.
Preheat the oven to 350°F.
Place the baking dish with the fish on the middle oven rack and bake for 12 to 15 minutes, until the fish flakes nicely with a fork.
Serve the fish topped with the sour cream and accompanied by the fennel.
If you cannot find aquavit, season the sour cream with 1/4 teaspoon ground fennel seeds, 1/4 teaspoon ground dill seeds, 1/4 teaspoon ground cumin seeds, 1 teaspoon sugar, and 1 tablespoon brandy.