Prepare the Vegetables. Wash and clean all the vegetables thoroughly. Cut the cauliflower into small florets. Slice the carrots, celery, bell peppers, garlic, and peppers into appropriate sizes.
Create the Brine. In a medium saucepan, combine the vinegar, water, and kosher salt. Bring the mixture to a boil, stirring until the salt is fully dissolved. Remove the saucepan from the heat and let the brine cool slightly.
Assemble the Giardiniera. In a large glass jar or multiple smaller jars, layer the prepared vegetables, starting with the cauliflower. Sprinkle dried oregano and red pepper flakes over the vegetables for added flavor and color. Tuck the garlic and onion in between the layers of vegetables.
Pour the Brine. Carefully pour the slightly cooled brine over the vegetables in the jars, ensuring that the vegetables are fully covered. Leave about half an inch of space at the top of the jars.
Seal and Refrigerate. Using a clean utensil, gently press down on the vegetables to release any air bubbles. Drizzle a thin layer of olive oil on top of the brine. This helps create an additional seal and prevents oxidation. Seal the jars tightly with the lids. Place the jars in the refrigerator.
Flavor Development. Allow the giardiniera to marinate in the refrigerator for at least 48 hours before enjoying it. The flavors will continue to develop over time.
Serve and Enjoy! Once the giardiniera has marinated to your liking, it's ready to enjoy! Store any leftovers in the refrigerator and continue to enjoy the giardiniera's bold flavors.