In a large saucepan combine crab stock with lemon grass and sugar cane. Cover and bring to a boil over high heat. When stock comes to a boil, remove from heat and leave covered for flavors to infuse, at least 45 minutes and up to several hours.

In a soup pot or large saucepan heat oil over medium-low heat. Saute onion, celery and garlic until vegetables are very tender, about 10 minutes. Strain infused crab stock over vegetable mixture and bring to a boil; adjust heat until soup boils gently. In a small cup whisk cornstarch with 1 to 2 tablespoons warm water until it forms loose paste. Whisk enough cornstarch mixture into soup to thicken slightly and boil 30 seconds. Turn heat to low and whisk in milk and cream. Simmer gently until heated through, but do not boil. Add crabmeat, season to taste with salt and pepper, and garnish with green onions, pepper flakes and shaved lemon grass. When crabmeat is warmed through, serve immediately.