Step 1: Prepare your jars. Wash two pint-sized glass jars (or one quart-sized jar) thoroughly with hot, soapy water, or run them through a dishwasher. For extra sterilization, you can boil them for 10 minutes if desired. Set aside.
Step 2: Slice the jalapeños. Wearing gloves, wash the jalapeños and then slice them into uniform 1/4-inch thick rounds. If you prefer less heat, carefully remove the seeds and membranes before slicing. Evenly distribute the sliced jalapeños into your prepared jars.
Step 3: Make the pickling brine. In a medium saucepan, combine the white distilled vinegar, water, pickling salt, and granulated sugar. If using, add the sliced garlic cloves, black peppercorns, and mustard seeds to the saucepan. Bring the mixture to a rolling boil over medium-high heat, stirring until the salt and sugar are completely dissolved.
Step 4: Pour the brine. Carefully pour the hot brine over the jalapeños in the jars, ensuring all the peppers are fully submerged. Leave about 1/2 inch of headspace from the top of the jar. Gently tap the jars on the counter to release any air bubbles. If needed, add more brine.
Step 5: Seal and cool. Wipe the rims of the jars clean with a damp cloth. Securely attach the lids and bands. Allow the jars to cool completely at room temperature for several hours. As they cool, a slight vacuum seal may form.
Step 6: Refrigerate and enjoy. Once completely cool, transfer the jars of pickled jalapeños to the refrigerator. While they can be enjoyed after a few hours, the flavors will develop and deepen significantly after at least 24 hours, and ideally a few days. Store your homemade pickled jalapenos in the refrigerator for up to 2-3 months.