Remove the outer leaves from each cauliflower head. Cut off the bottom stem end so that you create a flat base and can stand the cauliflower up on a cutting board. Resting the cauliflower on the stem, use a large, sharp knife to trim away the sides, then cut the remaining head into 2 very thick or 3 more moderate "steaks." Reserve the sides for another use (see notes). In a small bowl, stir together the olive oil, lemon zest and lemon juice, garlic, and honey.
Heat a grill to medium (about 350 degrees F). Brush one side of each cauliflower steak with the lemon–olive oil mixture and then sprinkle the brush sides with half of the salt. Place the salted side down on the hot grill, then brush the tops of the steaks with the olive oil mixture and season with the remaining salt. Cover the grill and let cook for 5 to 6 minutes, until the bottom is beginning to char. Flip the cauliflower, then re-cover the grill and cook 5 additional minutes, until the cauliflower is tender. Remove from the grill. Sprinkle with the red pepper flakes, parsley, and walnuts. Serve hot with lemon wedges.
The total cooking time will vary based upon how thick you slice the steaks, as well as your grill.
Ideas for leftover cauliflower florets: you can save them to roast or finely chop them and add them to a salad or stir-fry. If you prefer to cook them that night, you can also toss them with a little olive oil, salt, and pepper, then grill them in a grill basket and use them for salads, scrambled with eggs, or as a side.
HERE is a full list of additional cauliflower recipes, many of which are great ways to use up the leftover florets.