For the gremolata, combine the hazelnuts, lemon zest, parsley and a little seasoning in a bowl. Cover and set aside.
Toss the peppers, aubergine and courgette with 2 tbsp of the oil and the dried chilli flakes. Season well. Barbecue over white-hot coals for about 20 minutes, turning every so often, until charred and softened. Alternatively, preheat the oven to gas 7, 220°C, fan 200°C, and cook the vegetables in a roasting tin for about 30 minutes, until cooked through.
Meanwhile, warm the lentils through in a pan with 2 tbsp water. Combine the lentils with the cooked vegetables in a large serving dish. Drizzle over the lemon juice and remaining oil to dress the salad. Spoon over the gremolata to serve.