Step 1Blanch the spinach, watercress leaves, herbs and shallots in boiling water for 1 minute. Drain, refresh under cold water and squeeze to remove excess moisture.
Step 2Put them in a food processor or blender and add the gherkins, anchovies, capers, garlic, salt, pepper and cayenne. Puree to a smooth paste, adding one-fourth cup olive oil to help the mixture puree if necessary, then add the egg yolks and the butter, and process again until thoroughly mixed.
Step 3With the machine running, pour the remaining olive oil in a thin stream. The mixture should be glossy and as smooth as velvet. Add the vinegar and check the seasoning.