Prepare the spice crust: Pat salmon fillets dry with paper towels. Mix paprika, cumin, garlic powder, salt, pepper, and sesame seeds in a small bowl. Rub 1 teaspoon of olive oil on the flesh side of each fillet, then press spice mixture firmly onto the flesh side to form a crust. Set aside.
Make the yogurt sauce: In a bowl, combine Greek yogurt, dill, chives, green onion, red onion, and salt. Stir well and refrigerate until serving.
Cook the couscous: Bring vegetable broth to a boil in a saucepan. Stir in couscous and olive oil. Cover, remove from heat, and let sit 5 minutes. Fluff with a fork and mix in chopped parsley or chives. Keep warm.
Sear the salmon: Heat 1 tablespoon olive oil in a large nonstick skillet over medium-high heat until shimmering. Place salmon skin-side up (crust down) in the pan. Sear undisturbed for 3 minutes until crust is golden and crispy. Flip to skin-side down, reduce heat to medium-low, and cook 4-5 minutes more until skin crisps and internal temperature reaches 125F. Remove from heat immediately and let rest on a plate for 5 full minutes.
Plate: Spoon couscous onto plates. Top with salmon (skin-side down if preferred). Drizzle generously with herbed yogurt sauce. Garnish with sliced radishes.
