Each onion should be (topped and tailed), the outer papery skin removed,
then at least one layer below that.
It should be easy to tell as the outer layers will be slightly soft, just remove
those until you reach the firm onion.
Slice under water if onions make your eyes run, if this would be too difficult, just
soak the peeled onions in water for a few minutes before chopping, as this will
reduce any irritation considerably.