Heat the oven to 220°C/Fan 200°C/Gas 7. Set a large roasting dish on the top shelf to warm up.
Cut your squash into 1-2cm thick wedges. Leave the peel on if you like - it adds a bit of texture. Tumble it onto the roasting dish. Season well and dust with 2 tsp ras el hanut (more or less, to taste). Gloss with 1 tbsp oil. Roast for 30 mins or till golden.
While the squash roasts, halve, peel and thinly slice your onion. Set a large frying pan over high heat. Add 1 tbsp oil. Add the onion with a pinch of salt. Fry over medium-high heat till golden and crisp. Spoon onto a paper-lined dish to soak up excess oil.
Zest and juice half the lime. Pluck any woody stems from the watercress. Toss the leaves with the lime juice. Arrange half your watercress leaves on a plate. Dot the squash over. Arrange the remaining leaves over. Scatter over the lime zest.
Spoon the tahini over the top. Top with the onions and a pinch of ras el hanut. Serve with wedges of the remaining lime.