Want minced onion?
Chop top and bottom so onion sits flat and stable.
A large kitchen knife has the weight to do this. Razor sharp works best
Remove dried skin from outside
Start whacking the onion like you have a hatchet
Strong whacks. Sink the knife in over an inch
Make forty parallel cuts across the vegetable
Then rotate it ninety degrees
Start at the outside and work in -- both sides
Another forty parallel whacks
Turn onion on it's side and slice across the whacks -- thin slices
When you slice down into the yet to be whacked part of the onion start whacking again.
This is way faster than the old-fashioned way.
And remember tomates saladetes (Roma tomatoes) cut easier from the flesh to the skin.