Preheat oven to 375°F / 190°C (170°C fan). Cut the butternut squash in half lengthwise. Scoop out seeds and stringy bits. Rub the cut sides with olive oil, salt and pepper. Place cut-side down on a lined baking sheet and roast for 40–50 minutes, until the flesh is tender but the shell still holds shape. Cool 15 minutes.
While the squash roasts, cook the rice in salted water according to package instructions until just tender, 30–35 minutes. Drain well.
Once the squash halves are cool enough to handle, scoop out most of the flesh, leaving a ¾-inch (2 cm) wall so the shells stay sturdy. Mash the scooped flesh until smooth — you should have about 1 cup (8 oz / 225 g).
Heat 3 tbsp olive oil in a large skillet over medium heat. Cook onion for 5 minutes until softened, then add garlic, cumin, cinnamon, coriander, smoked paprika, and allspice. Cook for 30 seconds until fragrant.
Stir in rice, lentils, mashed squash, breadcrumbs, pine nuts, walnuts, parsley, salt and pepper. Mix until thick and cohesive. Taste and adjust seasoning.
Fill the roasted squash shells with the mixture, pressing down firmly. Place the halves open-side up on the baking sheet and bake for 25–30 minutes, until the filling is hot and set.
While the squash bakes, make the tahini sauce: whisk together tahini, lemon juice, garlic, olive oil, cumin, paprika, and salt. Gradually whisk in water until smooth and pourable. Adjust seasoning to taste.
Garnish the squash and serve with a generous pour of the tahini sauce and a dash of extra olive oil, then scatter with parsley, extra toasted pine nuts and a light sprinkle of sumac or zaatar. Let the stuffed squash rest for 10 minutes before slicing into wedges.