Roast the cauliflower: Preheat oven to 400°F/200°C. Toss cauliflower and onion with olive oil, smoked paprika, oregano, and salt in a roasting pan. Roast until golden-brown, about 20 minutes.
Cook the potatoes: Place diced potatoes in a medium pot and cover with water. Bring to a boil for 1 minute, then reduce heat to medium and cook until tender, about 20 minutes. Drain well.
Make the radish salad: Combine sliced radishes and parsley in a bowl. Toss with olive oil, lemon juice, za'atar, and salt.
Prepare tahini sauce: Whisk together tahini, lemon juice, and water until smooth and creamy. Add more water if too thick, or more tahini if too thin. Season with salt.
Finish the potato salad: Toss warm drained potatoes with scallions, olive oil, salt, and pepper.
Assemble: Serve components in warm pita pockets or arrange on plates. Drizzle generously with tahini sauce and add a dollop of harissa if desired.