Preheat the Oven: Preheat oven to 425°F and line a large baking sheet with foil or parchment.
Season the Vegetables: Place the carrots and red onion on the prepared baking sheet. Drizzle with olive oil and season with cumin, coriander, salt and black pepper. Toss well so the vegetables are evenly coated.
Roast the Vegetables: Spread the vegetables into an even layer and roast for about 30 minutes, until the carrots are tender and caramelized.
Make the Tahini Sauce: While the vegetables roast, whisk together tahini, garlic and lemon juice in a bowl. Add the water and continue whisking until the sauce loosens and becomes smooth and pourable. Season with salt and pepper. If needed, add more water 1–2 teaspoons at a time until you reach a thick, honey-like consistency.
Make the Pistachio Gremolata: In a small bowl, combine chopped pistachios, parsley, lemon zest, lemon juice and olive oil. Season with salt and pepper and stir to combine.
Assemble the Dish: Spread the tahini sauce onto a wide serving bowl or platter. Spoon the roasted carrots and onions over the sauce.
Garnish and Serve: Dollop the pistachio gremolata over the vegetables and finish with a sprinkle of Aleppo pepper. If desired, drizzle lightly with pomegranate molasses before serving.