Preheat the oven to 220C, gas mark 7. Cut the aubergines into 2cm chunks and scatter in 1 or 2 large roasting tins. Quarter the onions so they remain intact at the root ends and add to the tin. Drizzle with 3 tbsp of the oil and roast for 25 minutes. Stir in the peppers and return to the oven for a further 25 minutes, or until the vegetables are softened and lightly caramelised.
Mix the tahini paste in a bowl with the lemon juice, garlic and a little seasoning. Stir in about 3 tbsp water to give a smooth, creamy consistency.
Transfer the vegetables to a serving platter and drizzle with the remaining oil. Spoon the dressing over the top. Scatter with the parsley and lemon zest to serve.