Preheat the oven to 350°. Spread the olive oil on the space the trout will cover on a baking sheet. If it doesn't look like enough, use more. Better too much than too little.
Grind the hazelnuts. I put them in the food processor for a few minutes. You don't want them to be powdered.
In a shallow bowl, mix all of the ingredients except the olive oil and fish in a shallow bowl.
Rinse the fish. Do not pat dry. Coat the fillets in the hazelnut crust.
Place fillets skin side down on the oiled baking sheet. If you'd like more crust you can use the leftover from the bowl to add it to the top.
Bake at 350° until the fish is flaky. The amount of time will depend on the size of the fillet. A small trout will take ten minutes. A large salmon fillet could take 20-22 minutes.