After removing the outer leaves from each cauliflower head, cut off the bottom stem end and create a flat base. Resting the cauliflower on its stem, use a large knife to trim away the sides. Then, cut the remaining head into 2 very thick or 3 more moderate “steaks.” You can reserve the sides for another dish (see notes). In a small bowl, stir together olive oil, lemon zest and lemon juice, garlic, and honey.
Preheat grill to medium (about 350 degrees F). In a small bowl, mix together lemon and olive oil. Set cauliflower steaks on hot grill, Season with salt. Cover the grill and let cook for 5 to 6 minutes or until bottom is beginning to char.
Grill the cauliflower for 5 minutes, then remove the lid and flip it. After 5 more minutes have passed, check to see if the cauliflower is tender. If so, sprinkle on the red pepper flakes, parsley, and walnuts before serving hot with lemon wedges.
