Place lemon zest, herbs and spring onion into Thermo bowl and chop 3 sec/speed 6. Scrape down sides of bowl.
Add yolks, salt, mustard and vinegar to bowl and mix 1 min/37°C/speed 4.
Gradually add oil through hole in lid (holding MC in place), 3 min/37°C/speed 4.
Add juice and pepper. Taste and add salt if needed.
Blend 10 sec/speed 5.
Add avocado and blend 6 sec/speed 6. Scrape down sides of bowl and lid. Refrigerate unti use.