Get a large bowl, and then add your ice. Fill the bowl up with water and then add your sliced onions. Leave the onions in the bowl for about 5 minutes before straining.
Once you've drained the onions, put your red wine vinegar and a pinch of salt in a bowl, then add your onions, making sure they are coated in that vinegar.
Let the onion marinate in the red wine vinegar for a few minutes before draining on kitchen paper and then adding to the salad.
Put your extra virgin olive oil, red wine vinegar and a pinch of salt and pepper in a bowl and then whisk until well combined.
Store in something that allows you to easily spoon or pour the dressing over the salad (give the dressing a stir each time before you spoon/pour it over the salad).