Whisk in a small ladle of the warmed butter, a little at a time, then return the bowl over a gentle heat to cook a little more. Remove from the heat again and whisk in another ladle of butter. Repeat until all the butter is incorporated and you have a texture as thick as mayonnaise. Finally, whisk in lemon juice, salt and pepper to taste plus a little warm water from the pan if the mixture is too thick.
Set aside.