Rinse beans cold, drain. Peel onion and garlic and dice finely. Cut the avocado in half, remove the seeds and the flesh from the skin. For the topping, set aside approx. 1⁄4 avocado, beans and onion and mix with 1 tbsp lime juice and herbs. Season with salt and pepper.
Purée the rest of the avocado, beans, onion, garlic and the remaining lime juice. Stir in sour cream. Season to taste with salt, pepper and cayenne pepper. Arrange topping on top.