Heat sesame oil in a large soup pot over medium heat. Add lemongrass, ginger, lime zest, and snow peas and saute for 1 minute. Add bean sprouts and crab. Saute for 2 more minutes.
Add chicken broth and water. Bring to a simmer on medium heat and cook for 5 minutes. Remove from heat and add hot pepper sauce, sesame oil, and salt and pepper to taste. Ladle into 2 soup bowls and sprinkle scallions on top.