To be honest I've yet to try it but felt a need contribute a tip on onion prepping. Keep onions intact as long as possible. & 10-15 min in freezer will also aid in lessening that tears.
Pinch root 'whiskers', rub off as much dry skin as you can but try not to break through.
There is a thin film between the layers that interacts with the juices & creates the sulfuric fumes which cause tears. Try to prep onions as one of the last prep steps to minimize exposure to the fumes.
To minimize the tearing, only work with as many onions at one time that will fit on your cutting board when 1/2d. Be sure your knife is as sharp as you are comfortable with. Have a bowl large enough for finished amount at hand & a plate to cover the bowl.
Cut about 1/4" from the top end & peel back any remaining 'dry' layers, avoiding jagged edges in layers which will cause the aroma that causes the tears. Place cut end down & slice onion in half from root end down to cut end. Place onion halves cut face down on cutting board. Do this with 3-4 onions but leave room on cutting board for slicing.
Slice each onion. cut side down, to desired thickness but keeping slices touching until each 1/2 is done. Move sliced onion half to bowl without separating slivers if possible. Replace cover on bowl. Repeat as needed with skinned halved onions, then wipe down cutting boards before starting next batch.
Leek & scallion can benefit from similar steps & care, but unless using large quantities may not be worth the