In a small saucepan, melt butter until bubbly.
Place egg yolks, lemon juice, mustard, and seasonings into blender container; cover and blend on low until yolks are frothy.
Pour in half the butter in a very slow, steady stream until creamy.
Then turn blender to high and add remaining butter slowly.
Taste and correct seasonings.
Serve immediately on steamed vegetables such as Brussels sprouts, broccoli, zucchini.
If reheated, warm the sauce gently over hot water, stirring carefully.