Preheat your oven to 425°F (220°C).
Toss the cauliflower florets with ½ cup of olive oil, kosher salt, and freshly ground black pepper in a large mixing bowl.
Spread the seasoned cauliflower evenly on a large baking sheet. Roast in the preheated oven for about 25-30 minutes, tossing halfway through.
While the cauliflower is roasting, heat the remaining ¼ cup of olive oil in a skillet over medium heat. Add the sliced onions and sauté for about 10 minutes until soft and translucent. Stir in chopped garlic and oregano, cooking for an additional minute.
Once the cauliflower is done roasting, add it to the skillet with the sautéed onions, then gently fold in the sliced green olives and diced tomatoes. Cook for another 2-3 minutes until warmed through.
Transfer the mixture to a serving dish and sprinkle with chopped fresh parsley. Serve warm.
