Step 1: Wash the banana peppers thoroughly under cold running water. Pat them dry and trim off the stems. Slice the peppers into 1/4 to 1/2-inch thick rings. You can remove seeds if desired, though they are mild.
Step 2: In a large non-reactive pot (stainless steel or enamel), combine white vinegar, apple cider vinegar, water, granulated sugar, pickling salt, and mustard seeds.
Step 3: Add the smashed garlic cloves and bay leaves to the pot. Bring the mixture to a rolling boil over medium-high heat, stirring occasionally until the sugar and salt are completely dissolved.
Step 4: Carefully pack the sliced banana pepper rings into four clean, sterilized pint-sized canning jars. Pack them tightly but gently, leaving about 1/2 inch of headspace from the rim.
Step 5: Ladle the hot brine over the packed peppers, ensuring they are fully submerged. Use a non-metallic utensil (like a chopstick) to gently remove any air bubbles trapped within the peppers or jars.
Step 6: Wipe the rims of the jars clean with a damp cloth. Center the lids on the jars and apply the screw bands until they are fingertip tight.
Step 7: For shelf-stable storage, process the jars in a boiling water bath canner for 10-15 minutes (adjust time for altitude). Ensure the water covers the jars by at least one inch. If not canning, simply cool and refrigerate.
Step 8: Carefully remove the jars from the canner and place them on a wire rack or clean towel, spaced apart, to cool completely for 12-24 hours. Check for seals (lids should be concave and not flex when pressed).
Step 9: Store sealed jars in a cool, dark place for at least 2-3 weeks before opening to allow the flavors to properly meld. Refrigerate any unsealed jars or opened jars.