Preheat oven to 425°F (220°C). Toss cauliflower florets with 2 tablespoons of olive oil, salt, ground black pepper and rosemary in a bowl. On a baking tray lined with parchment paper, arrange cauliflower florets in a single layer. Bake for 15 minutes, or until the florets are tender and nicely browned. Flip the side halfway through.