If you are using regular cooking onion, then place it in a small bowl and pour boiling water over the onion and let stand for 20-30 seconds, then drain. This makes the onion sweeter.If you are using sweet or red onion then you can omit the boiling water step.
Drain the canned salmon and discard any skin or bones. Note, if you have cats then cats would love the drained salmon water.
In a large bowl mix together all the ingredients mashing the canned salmon and egg until a course pâté forms.