Put the pumpkin, courgettes and onion in a roasting tin with 2 tablespoons of the oil, season to taste with salt and pepper and stir to combine.
Roast in a preheated oven, 220°C (425°F), Gas Mark 7, for 25 minutes until all the vegetables are cooked.
Meanwhile, put the couscous in a heatproof bowl and pour over the measurement boiling water. Cover the bowl with a clean tea towel and leave to stand for 5 minutes, or until the grains are swollen and all the liquid has been absorbed.
Fork through the couscous to fluff up the grains, then stir in the roasted vegetables, cherry tomatoes and herbs.
Whisk together the remaining oil, the lemon juice and salt and pepper to taste in a small bowl and stir through the salad.