Finely chop the garlic clove. Add the salt and alternate between chopping it into the garlic and using the side of the knife blade to smash the salt into the garlic. Continue until the garlic resembles a paste.
Add the garlic paste and the juice of one lemon to a small food processor or to a blender.
Remove any larger stems from the herbs. Roughly chop the herbs.
Add the herbs, peeled and pitted avocado, sour cream, mayonnaise, and pepper to the processor/blender. Pulse to combine the ingredients; then blend on high until the dip is smooth.
Chill the dip for at least two hours (overnight is even better) before serving.
Check the flavor before serving. Add salt and pepper to taste.