In this crusty open-face sandwich, salmon fillets rubbed with a mixture of spices -- including cumin, fennel, thyme, oregano, and cayenne pepper -- are seared, then placed atop arugula and red onion.
Place cumin seed, fennel seed, oregano, and thyme in a spice grinder. Pulse until finely chopped but not powdery. Transfer to a small bowl; stir in paprika, cayennepepper, 1 1/2teaspoons salt, and 1/4teaspoon black pepper; set aside. In anothersmall bowl, whisk together horseradish, yogurt, honey, remaining1/2teaspoonsalt, and 1/4teaspoon pepper; set aside.
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.
The spice rub can be stored in an airtight plastic container for up to one month.