Prepare the water bath canner by filling it halfway with water and putting it on the stove to simmer.

Check three pint (500-ml) or six half-pint (250-ml) jars for any nicks or cracks, wash them in hot soapy water and rinse them in hot water. Keep the jars hot until it’s time to use them. Wash the lids in hot soapy water, rinse them, and set them aside.

Combine the salt, mustard, vinegar, water and sugar in a large stockpot and bring the mixture to a boil, stirring occasionally. Add the corn, peppers and garlic, and bring the mixture back to a boil. Reduce the heat to low and simmer the mixture for 20 minutes.

Remove the stockpot from the heat, and fill the jars with the corn salad, leaving 1⁄2 inch (12 mm) of headspace. Remove the air bubbles with a bubble remover tool or chopstick and recheck the headspace; add more brine if necessary. Wipe the rims with a clean cloth, put the lids on the jars and screw on the bands. Place the jars in the prepared hot water bath canner, making sure that the jars are covered by at least an inch (2.5 cm) of water.

Bring the water to a full rolling boil and put the lid on the canner. Process the jars for 15 minutes, adjusting for altitude, if necessary (see notes). Arrange a folded towel on the counter or table. Remove the jars, using a jar lifter, and place them on the towel. Let the jars cool for at least 12 hours.

Once the jars are cooled, remove the bands and check the seals. If any jars failed to seal, put them in the refrigerator to use first. Wipe the jars with a clean cloth and store them for up to a year.