The first thing you can do to avoid oily and runny mayo is to start with a good homemade mayonnaise recipe. However, even if you follow the measurements of the recipe to a tee, if you accidentally add the oil too quickly while whisking the emulsion won’t form properly and you’ll end up with an oily clear liquid instead of a thick white cream.
To fix this, add in another raw egg yolk and start whisking the mixture together. If the mixture starts to thicken after adding the yolk, that means it is working. Once this happens, you should start slowly adding more oil while continuously whisking. You will likely need to add quite a bit more oil and more seasonings because adding another egg yolk is essentially doubling the recipe. Although it sounds counterintuitive, adding more oil can also help thicken the mayonnaise. Also, if you end up with mayo that is too thick, you can always add a little water to thin it down.
However, if you add another egg yolk and the mixture still isn’t thickening, then the problem with the mayonnaise was probably nothing to do with the emulsion. It was likely the balance of other ingredients. If that’s the case, you can try some thickening methods that don’t involve eggs.
