Working over a bowl, use your fingers to squeeze the grated fresh ginger to release its juice. you should get about 1/2 to 3/4t of ginger juice. add the fish sauce if using, lemon juice, and crabmeat. gently mix.
In a wok or large saute pan, add the stock, sugar, salt, and pepper. bring to a boil. add the asparagus and immediately cover. steam on medium heat for 2 minutes, or until crisp-tender. use a pair of tongs to remove the asparagus to a plate, leaving the broth in the pan. drizzle the sesame oil over the asparagus.
Reutrn the pan with the broth to the stove on low heat. bring to a simmer and add the crabmeat mixture. return to a simmer. stir in the cornstarch slurry and let thicken, about 15 seconds. taste the broth, and season with additional salt if needed. pour over the asparagus.