Set an oven rack on the bottom level of the oven. Preheat the oven to 450°F. Line a roasting pan with foil. Stand cauliflower on the roasting pan, trimming the stem as needed so that it is level with the pan and the cauliflower will not tip over.

Whisk together the olive oil, 3 tablespoons Dijon mustard, salt, and pepper. Brush the entire outside, and inside crevices that can be reached, of the cauliflower.

Roast for 45 minutes, or until lightly charred and tender; a long skewer inserted in the center of the cauliflower should pass through easily.

While the cauliflower is resting, combine the parsley, Parmesan cheese, and the remaining 1 tablespoon Dijon mustard. Carefully but firmly press the parsley mixture onto the cauliflower.

Cut into wedges and serve hot.