Mix 1 tablespoon of the oil with the vinegar and lightly brush this marinade over both sides of the steaks. Crush the peppercorns and add the cumin seeds. Sprinkle the mixture over the steaks, pat it in gently and then place the steaks in a non-metallic dish. Set aside, or cover and refrigerate for several hours.
Preheat the oven to 200 degrees C. Put the shallots in a large, heatproof bowl and pour over enough boiling water to cover. Leave for 2–3 minutes, then drain. Peel off the skins, then return them to the bowl.
Trim the ends from the eggplants and cut them into 2.5 cm chunks. Add these to the shallots. Drizzle the remaining oil over the vegetables. Add the crushed garlic, then toss to lightly coat the vegetables with oil.
Heat a large, heavy-based roasting tin over a medium–high heat on the stove. Sear the steaks for about 1 minute on each side, then remove and set aside in the original marinade dish.
Tip the vegetables into the hot tin and spread them out. Place in the oven and roast for 15–20 minutes until softened. Lay the lamb steaks over the vegetables and drizzle any juices left in the dish over the lamb. Return to the oven to roast for 10 minutes.
Scatter the tomatoes into the tin and return to the oven for a further 8–10 minutes until the lamb is cooked to your liking and the vegetables are tender. Season to taste and serve sprinkled with chopped coriander.