Prepare Singapore Chile Sauce; set aside. Rinse crab back shell (if available) and set aside with crab.
In a small bowl, pound together garlic and ginger with a mortar and pestle (or mince very finely with a knife).
Heat oil in a wok or 5- to 6-quart pan over high heat. Add garlic-ginger paste; cook, stirring, until golden (30 seconds to 1 minute). Add Singapore Chile Sauce and crab; cook, stirring with a wide spatula, until crab is hot and sauce comes to a boil (3 to 4 minutes).
To serve, turn crab and sauce out of pan onto a platter; place crab back shell atop crab pieces. Garnish with cilantro sprigs, if desired. Serve with rice. Makes 2 servings. Singapore Chile Sauce. In a small bowl, smoothly blend 1 teaspoon cornstarch, 3/4 cup low-sodium chicken broth, and 1/4 cup catsup. Stir in 1 tablespoon fermented salted black beans, rinsed and drained (or 1 tablespoon reduced-sodium soy sauce), and 2 or 3 small fresh hot red or green chiles (such as Thai, serrano, Fresno, or jalapeno), seeded and minced.