Whisk together capers, rind, juice, oil, 1/2 tsp salt, garlic, and red pepper in a bowl.
Place beans in a bowl; drizzle with 2 Tbs caper mixture.
Heat a grill pan over medium high heat. Coat pan with cooking spray. Coat salmon with cooking spray; sprinkle with remaining 1/4 tsp salt and black pepper. Add salmon to pan, skin side down; cook 6 minutes. Turn salmon over; cook 1 minutes or until done. Keep warm.
Add arugula and onion to bowl with beans. drizzle with remaining caper mixture; toss. Divide salad among 4 plates; top each serving with 1 fillet. Serve immediately.