Preheat your oven to 375°F. Trim away the leaves at the bottom of the cauliflower head. Carefully cut off and remove the stem and tough core while keeping the head intact. Rinse the cauliflower well and pat dry with a clean towel.
Drizzle the olive oil all over the cauliflower and massage gently with your hands so that every part is coated. Season generously with kosher salt and black pepper if desired.
Place the cauliflower with its florets side up on a cast-iron skillet or oven-safe baking dish. Cover tightly with aluminum foil and roast for 30 minutes.
Remove the foil. Drizzle a little more oil if it looks dry. Roast for an additional hour, or until the cauliflower is tender and has a golden, slightly crispy exterior. Garnish with parsley.