Scoop out crabs, put the shells aside. Melt the butter in a saucepan, stir-fry the finely chopped onion until translucent, add the crab shells and continue to cook for approx. 2 min. Then extinguish with the white wine and 1/2 liter of water and simmer at medium heat for 1/2 hour, stirring occasionally.
Blend everything in a hand mixer and then pass through a fine sieve.
Preparation time: 50 min
Difficulty: normal