Make the mayonnaise:
In the bowl of a food processor, combine all ingredients except for the lemon juice (i.e., the basil, tarragon, chives, garlic, anchovies, lemon zest, salt, and mayonnaise) in the bowl of a food processor. Puree smooth, about 2 minutes, scraping down the sides of the bowl as needed. Blend in the lemon juice. Chill until needed.
Pickle the onions:
In a medium-sized jar, combine the onions slices, vinegar, sugar, salt, and peppercorns. Put the lid on the jar and let sit for at least 10 minutes, shaking a few times to dissolve the salt and sugar. Use immediately or refrigerate until needed, up to 1 week.
Assemble the sandwiches:
Toast the bread and spread both slices with a thick layer of the mayonnaise. Top with the avocado, mozzarella, tomato, cucumber, pickled onion, sprouts, and lettuce. Sandwich with the other slice of bread and press down gently. Optionally wrap the sandwich in parchment or secure with two toothpicks, and slice in half with a sharp serrated bread knife or chef’s knife.
