When I have lots of onions to slice, here’s a simple trick I use to avoid crying my eyes out.
Peel the onions, cut one in half, and wet the two pieces with cold water (I just hold them for a moment under the faucet). Shake off excess water and cut the halves into thin slices or diced pieces while they’re still damp. The film of water that clings to the surface of the onions will prevent (or at least reduce) the dispersion of the onion vapors that cause tearing. It may not work perfectly all the time, but at least you’ll be sniffling instead of bawling.