In a small mixing bowl whisk together the rice vinegar, water salt and sugar until salt and sugar have dissolved.
Place the coleslaw mix, onion slices and sliced fresh jalapeno into a large glass bowl, jar or dish with a tight-fitting lid.
Pour the rice vinegar mixture over the slaw and toss to coat. Cover and place the Curtido into the fridge to chill for several hours or overnight. This will last in the refrigerator for up to 1 week.