Peel the sweet potatoes and cut into 1 cm cubes. Peel the ginger and garlic cloves and dice them very finely. Finely chop the parsley.
Peel the onion, cut into wafer-thin rings and fry in butter in a saucepan until translucent, but do not brown, then lightly salt and pepper. Mix in the sweet potatoes, ginger, garlic and parsley and fry over a mild heat for about 15 minutes until the sweet potatoes are tender.
Pour 1 liter of water, add bay leaves and simmer the soup for about 15 minutes over medium heat. If you like it, you can use vegetable broth instead of water.
Peel, core and halve the avocado. Dice one half and add to the soup. Remove the bay leaves and put the soup in a blender and mix. Season to taste with salt, pepper and a little nutmeg.
To serve, dice the second half of the avocado. Pour the soup into preheated preserving jars, sprinkle with avocado cubes and sprinkle finely chopped parsley on top.