Place cumin seed, fennel seed, oregano, and thyme in a spice grinder. Pulse until finely chopped but not powdery. Transfer
Heat a heavy skillet over medium-high heat. Coat each fillet with 2 teaspoons of spice blend; pat with fingers. Place fillets in hot skillet. Cook until well browned, about 5 minutes. Flip fillets; cook through, about 5 minutes more. Transfer to plate; set aside.
To assemble, spread bread with horseradish mixture.Top with arugula and onion. Flake salmon into chunks; place on sandwiches. Drizzle with remaining horseradish mixture; serve.