Heat 1 tbsp olive oil in a skillet over medium heat. Pat salmon dry with a paper towel. Salt and pepper the salmon, then add to skillet, skin side up. Do not touch it for 12 minutes. It will form a perfect crust. Turn over and cook for 3-4 more minutes. Remove from pan to rest.
Toast pine nuts in a small pan over medium heat until just golden.
Heat remaining 2 tbsp olive oil in a separate skillet. Add onions and cook about 8 minutes until slightly browned and softened. Stir in vinegar, sugar, salt, plums, cherries, and capers. Cook until sauce thickens. Stir in butter.
Spoon onion sauce onto plates or a platter. Top with salmon fillets, skin side down. Sprinkle pine nuts and lemon zest over the top.
