For the herb butter, finely chop tarragon, parsley, basil and chervil. Cream the butter and stir in the finely chopped herbs.
Press the garlic and add the chopped onions as well. Season with salt, mustard and lemon juice and mix well. Now put the butter mixture on aluminum foil and form a roll with it.
Close it well at the ends and keep it in the refrigerator for a few hours. Cut into slices before serving.