To clarify butter; Melt 1/2 stick of butter over low heat. When melted, remove from heat and set aside for several minutes to allow the milk solids to settle to the bottom. Skim the clear (clarified) butter from the top, and discard sediment. (This can be done ahead of time).
To make the sauce: Heat clarified butter, add onion and garlic, and saute until transparent. Add lemon juice and white wine, and season to taste with salt and pepper. Simmer 2-3 minutes to reduce liquid. Remove from heat and swirl in cold butter until sauce is smooth and emulsified.