I use this for tuna, salmon and potato salad. It can be mixed half and half with store bought dressings like Miracle Whip as well.
2 tbsp sugar
2 tbsp flour
1-1/2 tsp dry mustard
1 tsp salt
Mix dry ingredients in a microwave safe bowl
Add to dry ingredients:
1 egg and blend to a smooth paste
Add 1 cup milk and stir until smooth
Gradually add 1/4 cup of vinegar
Cook for about 3 minutes in microwave (I use 50% power) and stir it every 30-60 seconds.
Cook until thickened. Add 2 tbsp of butter if desired. (I usually don't add butter)
Variations:
For fruit salads, add sugar or honey to taste and a squeeze of lemon juice. Fold in an equal amount of whipped cream.
For a mayonnaise-like dressing, drizzle 1/2 cup oil in cold basic dressing and whirl in blender at high speed.