Preheat a grill for cooking on high heat. Clean and oil the grates with vegetable oil.
Combine the vinegar, salt, sugar, oregano, peppercorns, garlic and 1 cup water in a small pot. Bring to a simmer, whisking occasionally, to make sure the sugar and salt are dissolved. Let cool completely.
Place the rainbow peppers, carrots, serrano peppers and cauliflower steaks directly on the grill (do not grill the celery; keep it raw!). Grill until charred with grill marks, about 5 minutes a side. Set aside to cool on a wire-racked sheet pan. Once cooled, it's time to chop!
Slice the carrots and celery 1/2-inch-thick on a sharp 45-degree angle bias. Keep the rainbow peppers and serranos whole. Cut the cauliflower florets off the woody stems and trim into individual 1-inch florets.
Place the chopped veggies into a 1-gallon resealable bag or container. Pour the vinegar mixture directly on the veggies. Stir around and let sit in the fridge at least 3 hours, but ideally overnight.
Drain the veggies from the brine and place on a paper towel-lined sheet pan. Reserve the brine. Arrange the pickled veggies in groups on a large, long platter, in other words, carrots, then peppers, then celery. Whisk together 3/4 cup of the brine with the olive oil and Dijon in a bowl until combined. Season to taste. Drizzle the vinaigrette over the veggies and top with the ricotta salata, celery leaves and olives. Or serve the veggies directly on freshly grilled Italian sausage or sausages sandwiches and top with ricotta salata, olives and celery tops!