Slice the aubergines about 2cm thick and sprinkle with salt. Stand for 10 minutes.
Heat your barbeque flat plate to a medium high heat.
Rinse eggplant to remove the salt and pat dry with absorbent paper. Add two tablespoons of sunflower oil to the flat plate. Cook the eggplant until dark brown and tender on both sides. If the eggplant absorbs all the oil before it is cooked then add a couple more tablespoons to the flat plate. Arrange on a serving plate and cover to keep warm.
Add another tablespoon of oil to the flat plate and add the onion and cumin seeds. Cook until just tender and slightly coloured. Arrange over the cooked eggplant. Dress the eggplant and onion with the yoghurt and sprinkle with ground cumin.
Slice the lamb back straps into 12 or 24 cubes (either 2 or 3 pieces per skewer). Pound the garlic clove in a mortar and pestle to form a smooth paste. Add the harissa spice mix and lemon juice and combine to form a paste. Rub the lamb pieces well with the spice mix and then skewer alternatively with lamb, tomato and lamb to use all the meat and tomatoes.
Cook your lamb skewers over a medium high heat until medium; remove from the heat and rest. Serve the skewers on top of the salad and scatter with the chopped herbs.