First peel the onions and cut them into cubes. Chop the walnuts as well.
Then melt the butter in a large saucepan over medium heat and sauté the onions until translucent. Stir in the flour. Add the walnuts and sweat with them. Now pour in the vegetable stock and simmer for 15 minutes.
Then pour in the cream and then finely puree. Pour in a little more stock or let it simmer a little.
Now salt and pepper the soup and add lemon juice to taste. Mix again until foamy and distribute in soup bowls. Garnish with a few walnut halves to round off.