For the topping, set aside about 1/4 of the avocado cubes, beans and onions and mix with 1 tablespoon lime juice and the herbs. Season with salt and pepper.
Mix the rest of the avocado, beans, onions, garlic and lime juice and puree with the blender. Stir in the sour cream and season with salt, pepper and cayenne pepper and spread the topping on top.