Check the crab meat to make sure there are no pieces of shell.
In a bowl, whisk together the mayonnaise, olive oil, lemon juice, salt, and cilantro. Mix the crab meat with the onion and the dressing.

Cut the avocados in half lengthwise and remove the pit. Use a spoon to carefully scoop out the whole avocado half in one piece. Slice a small flat section off the base of each avocado half so they sit flat on the plate.

Fill the avocado halves with the crab salad.
Refrigerate if not serving immediately.
You can serve the avocado on a bed of lettuce and with salads on the side.