Pour the lemon juice into a 2-quart sauce pan. Heat to medium-high. Using a whisk, stir the juice constantly until it is reduced to 1 tablespoon. Be careful not to brown the juice from overheating.
Divide the stick of cold butter into 8 pieces. Whisk 1 piece at a time over very low heat into the reduced lemon juice. If sauce starts to brown, elevate pan from heat and continue to whisk.
After all pieces of butter have been whisked, it’s ready to serve. If prepared in advance, it can be reheated by whisking 2-3 tablespoons hot water by driblets into the sauce, but do not refrigerate the sauce.
A great sauce for fish or veggies like broccoli and asparagus. Featured in Beurre au Citron ~ All-Purpose Lemon Butter Sauce.