Combine wine, vinegar, onion, salt, pepper, and parsley in a heavy saucepan; cook over high heat, stirring occasionally, until mixture is reduced to 1 1/2 tablespoons.
Reduce heat to low; add butter and yolks alternately, beginning and ending with butter. Whisk constantly until butter melts and mixture is smooth and thickened. Serve immediately.
Oxmoor House Homestyle Recipes
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