In a large mixing bowl, add 1 cup diced feta, 1 cup diced cucumber, 1 diced tomato, 2 sliced pepperoncini, red onion slices, 1 diced watermelon radish, 1/2 cup castelvetrano olives, and drained and rinsed chickpeas.
Add a large drizzle of olive oil (roughly 1/3 cup) and about 2 tablespoons of red wine vinegar. Add a pinch of dried oregano and some cracked black pepper. Mix contents of the bowl thoroughly.
Taste and add more olive oil, vinegar, or oregano as needed. Garnish with a pinch of flaky salt and a pinch of ground sumac.