Whip heavy cream in a small bowl until soft peaks form and et aside. Combine egg yolk and fresh lemon juice in a blender or food processor. Process until well mixed, then while blending, slowly pour in hot melted butter and continue blending until thick. Transfer to a saucepan and fold in whipped cream. Stir in chopped fresh dill (more or less to taste) and season with salt and freshly ground pepper. Keep warm over simmering water or very low heat. Serve with poached salmon or other meaty fish.