Prepare the pepperoncini peppers by washing them thoroughly under running water. Trim the stem end of each pepper, and if desired, you can make a small slit lengthwise on each pepper to allow the brine to penetrate further.
In a large saucepan, combine the white vinegar, water, sugar, salt, garlic cloves, black peppercorns, mustard seeds, dill seeds, coriander seeds, and bay leaf. Bring the mixture to a boil over medium heat, stirring occasionally to ensure the sugar and salt dissolve completely. Let the brine simmer for about 5 minutes.
While the brine is simmering, pack the pepperoncini peppers tightly into sterilized glass jars, leaving a little room at the top. You can stack them vertically or horizontally, depending on the size of your jars.
Carefully pour the hot brine over the peppers, ensuring that they are completely submerged. Leave a headspace of ½ inch in the jars.
Once filled, tap the jars gently on the counter to remove any air bubbles. Wipe the rims of the jars clean, then seal them tightly with the lids.
Let the jars be cool at room temperature. Once cooled, store them in the refrigerator for at least 24 hours to allow the flavors to develop. The pickled pepperoncini peppers will continue to improve in flavor over time and can be enjoyed for several weeks.