Preheat the oven to 180°C.
Cut the onion into wedges, the aubergines into half. Put them in a baking tray.
Mix the tomato purée with the oil, lemon juice, garlic, cumin seeds and marjoram. Add some water.
Pour this over the vegetables, making sure they are well coated. Scatter the preserved lemon around them.
Season with salt, chilli flakes and ground pepper, then cover the dish with foil.
Bake for about 25-30 minutes, then remove the foil. Add more water if needed and check the vegetables. If they are tender, scatter the broad beans around them and bake for another 5 minutes, uncovered. If they are not cooked through, then bake a bit longer, but this time uncovered, and add the broad beans when they've become tender.
Serve with chopped herbs scattered on top.