Place the zucchini and onions slices in the large bowl, add the salt, and toss to distribute. Add a few cubes of ice and enough cold water to cover. Stir to dissolve the salt, and allow to soak for about 1 hour.
After the 1 hour soak, taste and feel a piece of the zucchini. It should be softened and taste faintly salty. Drain well, and spin a few handfuls at a time in a salad spinner, or drain on towels.
Combine the vinegar, sugar, dry mustard, mustard seeds, and turmeric in the small saucepan and simmer for about 3 minutes. Remove from heat and allow to cool until just warm to the touch. The brine should not be too warm, or it will cook the vegetables and make them soft.
Pack the zucchini and onions into the two pint jars, then ladle the brine over the vegetables. Cover and refrigerate for at least one day before serving. Will keep in the refrigerator indefinitely.