Preparation time: a few minutes
Cooking time: about 20 minutes

1. Put the crabs into boiling water for about 5 minutes. Cut into quarters and sauté in hot oil over medium heat. 
2. Add the water and bring to a boil. Simmer for a few minutes, skimming the surface. 
3. Strain the broth and set the crab aside.
4. Pour the broth into a saucepan, heat for 5 minutes over low heat, add the soy sauce, very finely chopped lemongrass, grated ginger, chili and lime zest. 
5. Plunge the crab into this broth for 2 minutes. 
6. Cut up the sweet potato, previously cooked in water. Place into soup bowls and sprinkle with fresh mint.
7. Add the crab; pour the broth over top. Serve very hot.