Peel and stone the mangoes, then cut half of it into slices and the rest in cubes. In a mixing bowl, whisk together the egg yolk and the mustard. Slowly add the oil drop by drop to thicken the mayonnaise. Season the mayonnaise with salt and pepper and add the double cream, rum, curcuma and mango cubes. (Chili can also be added). Drain the crabmeat and then, mix it with enough mayonnaise to bind the mixture. Place a slice of the mango on a lettuce, and add a tablespoon of the crabmeat. Spread the remaining mayonnaise onto the crabmeat. Sprinkle with finely chopped onions.