1/2 cup (1 stick) unsalted butter, cut into cubes
1 medium shallot, finely minced
2 tablespoons whole grain mustard
2/3 cup Champagne vinegar
1 cup white wine
2 tablespoons freshly squeezed lemon juice
Salt and freshly ground black pepper
Set a medium sauté pan over medium-high heat. Add 1 tablespoon of the butter. Once melted, add the shallots and season with salt and pepper. Lower the heat and cook the shallots until translucent and soft. Add the mustard and cook down until dry. Add the Champagne vinegar and wine, then add the lemon juice. Let simmer until the liquids has reduced by three-fourths.
Lower the heat and while whisking constantly, add the butter, a couple of cubes at a time, incorporating fully prior to adding more. Reserve a teaspoon of butter to adjust at the end if necessary.
Blend with an immersion blender until frothy.
Season with salt and pepper. Keep in a small, covered pot in a warm place until ready to use. Note: It is important to keep this sauce warm, but not too hot, until you use it. If it gets too hot or too cold it will separate. Keep it over a pilot light and then warm it gently over a low flame, whisking constantly right before using.