Chop the Vegetables: Cut the cauliflower, celery, carrots, bell peppers, and mild peppers into small, uniform bite-sized pieces.
Brine Overnight: Place all the chopped vegetables in a large bowl and cover them with cold water and the ¼ cup of kosher salt. Cover and refrigerate for 12 to 24 hours to draw out excess moisture and crisp the vegetables.
Rinse: Drain the vegetables thoroughly in a colander and rinse them well with cold water to remove excess surface salt. Pat them dry with paper towels.
Mix the Marinade: In a separate bowl or measuring cup, whisk together the olive oil, white wine vinegar, minced garlic, dried oregano, celery seed, and black peppercorns.
Combine and Store: Place the drained vegetables and optional olives into clean glass jars. Pour the oil-and-vinegar mixture over the vegetables until they are completely submerged.
Marinate: Seal the jars tightly and store them in the refrigerator for at least 24 to 48 hours before eating so the flavors can fully develop. Keep refrigerated with the vegetables submerged in oil.