Preheat oven to 350°F.
In a small bowl, combine the horseradish and mayonnaise. Set aside.
In the bowl of a food processor, crush nuts until they are coarsely ground but not too fine. Add in brown sugar and lemon juice and pulse until ingredients are thoroughly combined.
Place a salmon on a large parchment lined baking sheet. Spread horseradish-mayonnaise mixture over the fish. Top with nut mixture.
Bake for 25 minutes, until fish flakes easily with a fork.