Preheat the oven to 425°F.

Make the sauce: In a small bowl, mix together the harissa, olive oil, cumin, paprika, thyme, salt, and red pepper flakes.

Slice the bottom of the core off the cauliflower so that it sits flat. Place the cauliflower in a cast iron skillet and brush the cauliflower with 1/3 of the sauce. Cover the skillet with foil and roast for 30 minutes. Uncover and roast for 30 more minutes, or until the cauliflower is fork-tender in the middle.

Remove from the oven and brush with half the remaining sauce. Season with salt and pepper and squeeze with lemon juice. Garnish with cilantro, slice into quarters, and serve with the remaining sauce on the side.