Preheat the oven to 450ºF / 230ºC degrees.
Trim the cauliflower and make a shallow ridge down the center to create the appearance of two brain lobes. Be careful not to cut too deeply, as you want to keep the head intact.
Mix together the olive oil, sage, red pepper flakes, and salt and pepper. Brush this mixture all over the cauliflower, rubbing it in well to coat every crevice.
Place the cauliflower in a baking tin, cover with foil, and roast for 50 minutes until tender.
Remove from the oven and turn the broiler on.
Uncover and sprinkle the parmesan over the top, then broil for 3–4 minutes until golden.
Meanwhile, warm the marinara sauce in a pan.
To serve, spread some marinara sauce onto a platter and place the cauliflower on top. Pipe veins over the surface of the cauliflower using a piping bag filled with marinara for a realistic “brain” effect.