Melt the butter in a large saucepan, add the leek and sweat, it must not take any color. Then add the potatoes and let them sweat for a moment. Deglaze the vegetables with the meat stock, season with salt and pepper. Add the frozen herbs or the fresh herbs (tied into a bouquet of herbs, removed after cooking). Let the soup simmer for about 35 minutes until the potatoes are cooked, then puree.
Add the crabs and let stand for about 10 minutes.
Finally stir in the sour cream and serve the soup sprinkled with chives.
I always cook the soup the day in advance because it tastes even better when it is pulled through.