Heat oven to 425°F Spray 11x7-inch glass baking dish with cooking spray.
Heat oil in large nonstick skillet over medium-high heat until hot. Cook onions and 1/4 teaspoon of the salt 3 to 4 minutes or until onions begin to turn golden brown, stirring frequently. Reduce heat to medium-low and add garlic; cover and cook 5 minutes or until tender, stirring occasionally.
Meanwhile, combine orange juice, vinegar, orange peel and rosemary in small bowl; reserve 2 tablespoons for glaze. Add remaining mixture to onions; cook, uncovered, 2 to 3 minutes or until most of the liquid has evaporated. Whisk mustard and honey into reserved glaze.
Spread onion mixture in baking dish; top with salmon. Sprinkle with remaining 1/4 teaspoon salt; spoon reserved glaze over salmon. Bake 8 to 10 minutes or until salmon just begins to flake. Serve salmon topped with onion mixture.