Combine first 5 ingredients in a small saucepan. Cook over medium-high heat 4 minutes or until sauce thickens, stirring constantly. Remove from heat; keep warm.
Heat 1 teaspoon oil in a large nonstick skillet over medium-high heat. Add onions; sauté 3 minutes or until tender. Remove from pan; keep warm.
Heat 1 teaspoon oil in pan over medium- high heat. Sprinkle fish with salt. Add fish to pan, skin side down. Cook 6 minutes on each side or until lightly browned and fish flakes easily when tested with a fork. Divide onions evenly among 4 plates. Place fish over onions; drizzle with mustard glaze.