If the mayonnaise separates after the oil is added, the mixture has broken. To bring it to a creamy state, add an egg yolk and 1 teaspoon tepid water to a clean bowl, and slowly add the broken mayonnaise while whisking constantly. Then, whisk in 1/4 cup more oil.
I used to snicker to myself - who would MAKE mayo? - until my jar was empty and I wanted to make an egg salad sandwich. With simple ingredients already on hand I decided to give it a whirl in my Cuisi mini prep. How easy! Now I'll snicker to myself - who would BUY mayo?