Using a blender or electric mixer, blend egg yolks, salt, sugar and 1 tablespoons of the lemon juice.
Slowly pour in 1 cup oil, a few drops at a time, mixing thoroughly.
Add remaining ½ cup oil a little faster, carefully blending in each bit before adding the remaining 2 tablespoons lemon juice.
Mix the flour, mustard, boiling water and vinegar together until smooth.
Pour into a small saucepan and cook until smooth and paste-like (but not too thick).
Slowly add this hot mixture to the mayonnaise and blend well.
Pour in a container and cool in the refrigerator.
