Combine cauliflower, carrots, celery, bell pepper, garlic, and serrano in a large bowl. Toss to combine, then pack into 3 pint-sized glass jars.
In a medium saucepan over medium heat, combine vinegar, water, sugar, salt, bay leaves, fennel seeds, coriander seeds, and dried oregano. Stir to dissolve salt and sugar, and bring up to a boil. When mixture is boiling, remove from heat, remove bay leaves, and pour mixture over jarred vegetables. Let cool completely, lid, and refrigerate. Giardiniera will last at least 1 week in the fridge.
