Combine wine, vinegar, and onion in a small saucepan. Cook over medium heat 10 minutes or until liquid is reduced to 1/2 cup. Strain; discard onion. Cool slightly.
Beat egg yolks until thick and lemon colored. Gradually stir one-fourth of hot mixture into yolks; add to remaining hot mixture, stirring constantly. Cook, stirring constantly, until thickened. Remove from heat; stir in butter, 1 tablespoon at a time. Stir in remaining ingredients. Serve immediately.