Slice around the avocado pit lengthwise, twist apart halves, remove pit, and scoop meat out of the skins. Add the avocado meat to a bowl and mash. Add lemon juice and stir.

Slice crabmeat into small chunks. Dice the onion very finely. Slice the cucumber segment in half, scoop out the seeds with a grapefruit spoon, and slice each half into thirds before dicing into small sections. Add these cut things to the bowl, as well as the mayo, soy sauce, and sesame oil. Mix together well and serve atop the bagels.