I had my mandolin slicer out the other day to slice up cabbage and onions for my coleslaw.
While I had that out I sliced up another 3 big onions to keep in the fridge so that I’d not have to do the whole cry while peeling onions again in the week.
My sliced onion was put in a sealed container that was lined with kitchen paper towel, I find it keeps in the fridge for up to a week.
You can also slice up a few carrots this way and keep in the fridge for a couple of days as a quick snack.
I’ve also frozen sliced onion but I find like capsicum it’s only usable for casseroles or crockpot cooking as it doesn’t defrost as nicely for a salad, but ultimately that is up to you.