Preheat oven to 400°.
Place potatoes in a saucepan, and cover with water; bring to a boil. Reduce heat, and simmer for 15 minutes or until tender, and drain. Return potatoes to pan. Add buttermilk, 1/2 teaspoon salt, 1/4 teaspoon black pepper, 1/2 teaspoon lemon rind, and juice; mash with a potato masher. Keep warm.
Combine panko, onion, dried oregano, paprika, and 1/4 teaspoon pepper in a shallow bowl. Sprinkle salmon evenly with remaining 1/4 teaspoon salt. Dredge fillets in panko mixture.
Heat 2 teaspoons oil in a large nonstick ovenproof skillet over medium-high heat. Add salmon to pan; cook 2 minutes on one side or until browned. Turn fish over; place skillet in oven. Bake at 400° for 6 minutes or until fish flakes easily when tested with a fork or until desired degree of doneness.
While salmon cooks, heat remaining 1 teaspoon oil in a large nonstick skillet over medium heat. Add garlic to pan; cook 1 minute, stirring constantly. Gradually add spinach, turning frequently; cook for 3 minutes or until spinach wilts. Stir in remaining 1/2 teaspoon rind and remaining 1/4 teaspoon pepper. Arrange 1/2 cup spinach mixture on each of 4 plates; top each with 3/4 cup potato mixture and 1 fillet. Garnish with oregano sprigs, if desired. Serve immediately.