If the nopales you buy do not already have the thorns removed, do so carefully. I recommend using gloves and a carrot peeler to do this. Once you have removed all of the thorns, wash and chop the nopales, place them in a pan with the 4 cups of water, halved onion and salt. Bring to a boil, reduce heat and simmer 30-45 minutes or until tender. Drain nopales and combine with remaining ingredients. Taste and adjust seasoning adding more salt if necessary. This dish gets better if you let it sit a few hours in the fridge before serving.