Peel the potatoes, carrots and onions and cut into cubes. Clean and wash the leek and cut into rings. I don't use the green part of the leek, otherwise the soup will be very dark.
Heat the butter lard in a large saucepan and sweat the onion cubes in it over medium heat until translucent. Add the rest of the vegetables and sauté over low heat for about 10 minutes.
Pour in the broth and cook the vegetables until soft in 20 minutes.
Add the cream, bring to the boil briefly and puree with a hand blender. Season to taste with nutmeg, salt and pepper. Keep warm.
Wash and clean the spring onions and cut diagonally into fine rolls. Heat the butter in a pan and toss the spring onions in it. Add the crabs and heat.
Pour the soup into deep plates and add the crab and spring onion mixture.