1) Preheat the oven to 200°C/gas mark 6.
2) Cut the aubergines, red pepper and onion into 2-cm cubes. Toss them in a large bowl with the garlic, olive oil, cayenne, salt and pepper. Spread on a baking tray and roast for 45 minutes, until the vegetables are lightly browned and soft, tossing once during cooking. Cool slightly.
3) Place the vegetables in a food processor fitted with a steel blade, add the lemon juice and tahini, and pulse three or four times to blend. Taste and add salt and pepper as needed. Transfer to a bowl and add the chopped parsley.
Garnish with the extra parsley and serve.