Prepare the Vegetables: Wash and peel the vegetables as necessary. Slice the carrots, celery, peppers, and onion into bite-sized pieces. Separate the cauliflower into small florets. If using, slice the hot peppers into thin rounds. Mince the garlic.
Sterilize the Jars: Before starting, sterilize your jars by placing them in boiling water for 5-10 minutes, then allowing them to dry completely.
Prepare the Brine: In a medium saucepan, combine the white vinegar, water, salt, sugar (if using), oregano, mustard seeds, black peppercorns, celery seeds, and bay leaf. Bring the mixture to a boil, stirring until the salt and sugar have dissolved. Once boiling, reduce the heat and simmer for 5 minutes to allow the flavors to meld together.
Pack the Vegetables: Layer the prepared vegetables into the sterilized jars, packing them tightly but not too tightly to allow for expansion. Add minced garlic and optional hot peppers between the layers.
Pour the Brine: Carefully pour the hot brine over the vegetables in the jars, ensuring they are completely submerged. Leave about 1/2-inch space at the top of the jar.
Seal the Jars: Wipe the rims of the jars with a clean cloth and seal with the lids. Allow the jars to cool to room temperature before refrigerating.
Marinate: Let the giardiniera sit in the refrigerator for at least 24 hours before eating to allow the flavors to fully develop. It’s even better if allowed to marinate for 3-4 days.