Red onions can be incredibly harsh. I had a restaurant salad once with red onions that left my nose and sinuses burning for hours – and I absolutely love onions.

This is a nice chart, but red onions are the sharpest of them all, IMO.

If you’re using red onion raw and it won’t be cooked in the final dish, it always pays to taste a bit before adding the rest of the red onion to whatever you’re making: if it’s too sharp/piquant for comfort you can soak the cut pieces in cold water for a few minutes and then drain and dry before using to bring them back into comfortable eatability.