Béarnaise Sauce

Put onions, vinegar, tarragon and pepper in a sauce pan and bring to a boil. Reduce to about 1-2 teaspoons of liquid. Stir constantly. Remove from stove and let cool. Put egg yolks and cooled mixture in a blender. Process for about 1 minute or until completely blended. Melt butter. Turn blender to high and slowly add the butter. Start out very slow. Serve immediately. This can be a tricky sauce so go slow and watch it very carefully. The sauce will get thick as you add the butter. Do not let it cool after it is mixed because it will separate.