Mayonnaise can be tricky to make and can separate into oils and solids. It should not smell unpleasant, though it may be a bit more sour and may look a little yellow. It is still edible, however, as long as it does not smell unpleasant and it has not been subjected to extremely high temperatures.
Blender or food processor, freshly washed in very hot water so that it is still warm.
Slowly pour the turned mayo into the glass measuring cup. Don’t stir it up too much.
Begin by pouring a very small amount of mayonnaise, a little over a tablespoon, into your warmed blender or food processor. Add 1 teaspoon of mustard and process the mixture until it becomes creamy again.
Repeat by pouring in the same small amount of turned mayonnaise, then the mustard, and processing until thickened again. You can reduce the mustard as you go if you like, but try to keep the area you are working in warm so that the oils within the mayonnaise begin to bond again.
Keep doing this until your mayonnaise has bonded again and you have used up all your turned mayonnaise.
You can also use this technique if your mayonnaise did not turn out well when you first made it. Just remember to keep the utensils warm so that the oils are not too cold to be placed in suspension in the mayonnaise.