Peel and chop the onions. Peel and finely chop the garlic. Heat the oil in a pot, fry the onions and garlic. Remove 1 tablespoon of peas and put aside. Add the remaining peas and fry briefly.
Deglaze with stock, bring to the boil and simmer at medium heat for about 8 minutes. In the meantime mix crème fraîche and horseradish. Cut the eel into small pieces. Remove peas from the heat and puree them finely in the stock with a blender.
Add the rest of the peas and bring to the boil again briefly. Season with salt and pepper. Serve the soup with some horseradish crème fraîche, some pieces of eel and, if desired, garnished with mint.