1. First, make the green mayo: place the egg yolks, mustard, vinegar and salt into an electric beater or blender and blend briefly for about 15 seconds until mixed.
2. Combine the two oils together in a jug and with the beater or blender running, pour the oils through the funnel in a long thin stream until the mixture thickens and emulsifies.
3. Stop the machine, add the herbs and watercress and sour cream and blend again until well mixed. Pour into a clean bowl and cover with cling film. Chill and use the same day.
4. For the salad, place the green and red sliced apples in a large mixing dish with the lime or lemon juice to prevent them from browning. Add the celery, fennel bulb slices, herbs and mix well.
5. Pour enough mayonnaise to coat everything and mix. Tip into a serving platter or bowl and garnish with extra fennel fronds.
I mixed the olive oil with canola oil for the green mayo to neutralise the pungent and slightly bitter flavour of the olive oil.
The sour cream is an option but adds to the thickness, texture and taste of this mayo.
If you are sensitive to raw egg yolk then it's best to use an egg free mayo blended with fresh herbs.
Unlike most salads where you add the dressing just before serving, you can add the green mayo dressing way ahead of serving time for this salad. Just remember to keep the salad in the fridge once the mayonnaise has been added.