Remove the outer leaves from each cauliflower head and use a large, sharp knife to trim away the sides, and slice into 2-3 pieces.
In a small bowl, stir together the olive oil, lemon juice, garlic, agave syrup, and sea salt. Brush the cauliflower steaks with this mixture.
Cover the grill and let cook over medium heat for 5 minutes, until the bottom is beginning to char. Flip the cauliflower, then re-cover the grill and cook 5 additional minutes, until tender. Remove from grill and sprinkle with the red pepper flakes and parsley. Serve hot.