Mayonnaise is such a blessing in the hot summertime, it makes cold food so much more delicious, even cold fish becomes delectable when served with a herb mayonnaise, and cold chicken is definitely at its best with mayo.
This recipe can be varied by adding more or less mustard, and by altering the proportions of olive oil and sunflower oil. Adding more olive oil gives a richer, heavier result. Some people find mayonnaise made entirely with olive oil tends to be too strong and even rather bitter.
Add crème fraîche, as much as you like, to lighten the mayonnaise, and, of course, add chopped herbs. Parsley, chives and chervil, plus a few chopped capers, is a fresh combination for fish.