Peel the avocado. Using a wide knife, cut the avocado lengthwise to the pit, grab the berry with both hands and twist in opposite directions to separate one half from the pit. Use a tablespoon to separate the flesh from the skin. This should be easy if the fruit is ripe. Cut the avocado into cubes.
Chop the capers.
Cut the salmon fillet into cubes.
Finely chop the onion.
In a deep bowl, combine salmon, onion, capers, soy sauce and vegetable oil.
Add the avocado there and stir again.
Serve the salmon tartare on a flat plate. Sprinkle the dish with lime juice before serving.