Preheat the oven to 400ºF. Place the cauliflower on a rimmed baking sheet. Drizzle over 2 tablespoons of the olive oil, sprinkle with the paprika, ½ teaspoon cumin, and cayenne, and season with salt and pepper. Use a spoon or your hands to toss everything until the cauliflower is well coated with the seasoned oil. Roast for 20 minutes until the cauliflower is starting to soften, then add the chickpeas to the pan, use a spoon to stir so that they are evenly distributed and coated with the seasoned oil, and roast for another 15 minutes. Let cool to slightly warm, or room temperature.
Meanwhile, make the dressing. Place the tahini, water, remaining 2 tablespoons olive oil, lemon juice, garlic, remaining ½ teaspoon cumin, and salt and pepper in a food processor or a blender, and process until thick and smooth. You can also combine all of the ingredients in a container or bowl and shake or stir to blend well.
Place the slivered romaine hearts in a shallow bowl, and top with the cauliflower and chickpea mixture. Spoon the thick dressing over the top, and toss to coat the vegetables with the dressing. Top with the onions and parsley leaves.