With a knife or food processor, thinly slice the onions. Scoop into a non-reactive bowl. Pour boiling water over them, count to 10, then immediately pour the onions into a strainer. Shake off all the water, pour the onions back into the bowl, pour on the lime juice and stir in the 1 1/2 teaspoons salt. Cover and refrigerate for at least 1 hour. The onions will last for a week or more in the refrigerator.