See above Ingredient Substitutions section for types of onions to use. However, if you absolutely cannot use onions, then here are a few alternatives. You could use diced cucumber, red bell pepper, or green bell pepper. These will not give you the same flavor, but will still give the crunch. You could also mince a garlic clove into this. This will be more for the pungent flavor vs. texture, so you will need to add either more tomatoes or one of the other alternatives.