Puree the egg, garlic, lemon juice, parsley and spring onions for about 15 seconds in a food processor or with a magic wand. Then slowly but steadily add the olive oil. Mix in the cayenne pepper and mustard well, season with the salt. Cover and let the sauce rest in the refrigerator for 1 hour.
Consume within the next 24 hours. The sauce goes well with fish, shrimp and pan-fried meat. I also like to spread it on my baguette.