Heat Wave (Preheat & Pan Prep) : Preheat oven to 450°F (232°C). Line a rimmed sheet pan with parchment and brush with 1 tbsp olive oil.
Flavor Stir (Za’atar Drizzle Base) : In a small bowl, whisk 3 tbsp olive oil with the garlic, za’atar, sumac, Aleppo, lemon zest, and black pepper; set aside so the spices bloom.
Bubble Bath (Par-Boil for Fluff) : Bring 3 quarts / 2.8 L water to a boil in the Dutch oven. Add 1 tbsp kosher salt, the baking soda, then the potatoes. Simmer 15–18 min until a skewer slides in with gentle resistance.
Steam-Off (Dry = Crunch) : Drain well, return potatoes to the hot pot, and shake gently to rough the surfaces. Spread on the sheet pan (or a rack set over it) and air-dry 12 min until skins look matte.
Press & Dust (Texture Build) : Smash each potato to about ½ in / 1.25 cm thick—keep them in one piece with plenty of craggy edges. Sprinkle with ¾ tsp kosher salt; dust lightly with 1 tbsp cornstarch (optional). Drizzle with the remaining 2 tbsp olive oil.
Roast to Ridges (Crisp Finish) : Roast on the middle rack 30–35 min, rotating the pan once, until deeply golden with rugged, lacy edges and tender centers.
Warm Pour & Toss (Gloss and Glow) : Whisk ¼ cup (60 mL) hot reserved potato water into the spice oil to loosen it so it coats evenly. Spoon over the hot potatoes. Shower with parsley, green onions, and a pinch more za’atar. Serve immediately with lemon wedges, if desired.