Blackened Salmon Sandwiches

Name: Blackened Salmon Sandwiches
Ingredients: 1 teaspoon cumin seed; 1 teaspoon fennel seed; 1 teaspoon dried oregano; 1 teaspoon dried thyme; 2 teaspoons paprika; 1/2 teaspoon ground cayenne pepper; 2 teaspoon coarse salt; 1/2 teaspoon freshly ground black pepper; 2 tablespoons prepared horseradish; 1/4 cup plain low-fat yogurt; 1 tablespoon honey; 4 four-ounce salmon fillets, skin removed; 4 slices crusty bread; 1 bunch arugula, stems trimmed; 1/2 small red onion, thinly sliced crosswise