Heat the olive oil in a large pot over medium-low heat. Add the fennel and leeks and season with 1 1/2 teaspoons salt and the red pepper flakes. Cook, stirring occasionally, until the vegetables are just tender, about 25 minutes.
Add the Sambuca and cook until evaporated, about 1 minute. Add the tomatoes, chicken broth, heavy cream and ½ cup water. Bring to a simmer over medium heat; cook until the fennel is soft, about 15 minutes.
Working in batches, transfer the soup to a blender and puree until smooth (or puree in the pot with an immersion blender). Return the soup to the pot; return to a simmer and season with salt. Top each serving with the crab and fennel fronds.