Toss the sliced cauliflower with the butter and olive oil. Season with salt and black pepper. Transfer the cauliflower to a baking sheet, in single layer, roast until almost tender about 20-30 minutes. Remove from the oven and sprinkle the grated Parmesan and chopped parsley leaves on top of the cauliflower, roast again until the cheese melts and slightly crusty, about 5 minutes. Remove from oven and serve immediately.