Mix water and kosher salt in a medium bowl. Add cauliflower, celery, carrots, serrano peppers, bell peppers, onion, garlic, and green olives. Toss to combine and refrigerate overnight.
Drain vegetables and rinse very well. Place in a large jar.
Whisk together vinegar, olive oil, oregano, and chili flakes. Pour over the vegetables and refrigerate at least 3 days before eating.
Keep up to 1 month.
