Mix half the lime juice with the ginger and lemon grass. Add the mayonnaise and mix well.
Spread 1 tablespoon mayonnaise over each slice of bread.
Halve the avocado and remove the pit. Peel and slice the flesh thinly, then arrange the slices on the bread. Sprinkle with lime juice.
Spoon the crab meat over the avocado, then add any remaining juice. Spoon over the remaining mayonnaise, top with a cilantro spring.
*** To make lime-and ginger-flavored mayonnaise, place 2 egg yolks, 1 tablespoon lime juice, and 1/2 teaspoon grated ginger in a blender. With the motor running, gradually add 1 1/4 cups olive oil, drop by drop, until the mixture is thick and smooth. Season with salt and pepper.***.