Combine the cauliflower florets, carrot coins, celery slices, bell pepper pieces, sliced serrano peppers, and pitted green olives in a large heatproof bowl.
In a medium saucepan, combine the red wine vinegar, pure olive oil, sugar, minced garlic, dried oregano, salt, and black pepper. Bring the mixture to a simmer over medium-high heat, stirring until the sugar dissolves completely.
Carefully pour the hot vinegar brine over the vegetables and olives in the bowl. Stir the mixture thoroughly to ensure all the vegetables are evenly coated in the brine.
Allow the mixture to cool at room temperature for about 1 hour, stirring occasionally. Then, cover the bowl tightly with plastic wrap and refrigerate for at least 24 hours, and up to 5 days, to let the flavors meld and the vegetables crisp-tender.
Before serving, give the giardiniera a good stir. Use a slotted spoon to transfer the pickled vegetables and olives to a serving bowl, leaving the excess brine behind. Serve chilled or at room temperature.
Notes
Let giardiniera rest at least 24 hours for best flavor. Store in refrigerator for up to 5 days.