In a large bowl, layer the crab meat in one layer along the bottom.
Sprinkle the thinly sliced onions on top of the crab meat.
Pour the oil on top of the onions and crab meat.
Pour the cider vinegar and the water on top of the onions and crab meat.
Squeeze the juice from one-half of a lemon (careful to remove any seeds) all over the top of the other ingredients.
Add salt and pepper (you can add more to taste)
Refrigerate for at least 6 hours covered. Serve cold either by itself or on a bed of lettuce.