Preheat oven to 450 degrees. In large skillet, over high heat, cook salmon fillets 2 minutes each side or until well browned. With spatula, carefully transfer fillets to jelly-roll pan. Set aside. With paper towels, wipe out skillet. Add onions; over medium-high heat, stir-fry 5 minutes or until caramelized. Add jalapeno, plums, 1/2 teaspoon salt, 1/4 teaspoon pepper, the vinegar and 1 tablespoon of water. Cook 2 to 3 minutes or until onions are tender. While onions cook, bake salmon 3 to 4 minutes or just until cooked through. Serve over onion mixture.