In a large bowl, combine 2 Tbsp. of the oil, 1/2 tsp. salt, and 2 Tbsp. harissa; mix well. Add cauliflower and toss to evenly coat. Spread on a rimmed baking sheet and bake for 35 minutes, tossing once halfway through, until tender. Remove from oven and toss with remaining 2 Tbsp. harissa; bake for another 5 to 10 minutes, until caramelized and charred in some spots.
Meanwhile, prepare Tahini-Honey by combining remaining 2 Tbsp. oil, 1/4 tsp. salt, tahini, honey, lemon juice, and black pepper; stir with a whisk. Whisk in 1 Tbsp. water to thin out to desired consistency.
Transfer baked cauliflower to a bowl or serving platter. Drizzle with tahini-honey, and garnish with fresh parsley, if desired
Store in an airtight container and refrigerate up to 3 days.