Preheat the oven to gas 6, 200°C, fan 180°C. Lightly oil 2 large roasting trays and then divide the onion, courgette, aubergine and peppers between the trays. Add the garlic and rosemary, season well and drizzle with the rest of the olive oil. Roast for 25 minutes, switching the trays over after half the time has passed, so that the vegetables cook evenly. Add the cherry tomatoes and balsamic vinegar and roast for another 5 minutes.
Meanwhile, preheat the grill. Mix the breadcrumbs, chopped rosemary leaves, feta, olive oil and some seasoning in a bowl. Season the lamb steaks on both sides and grill on one side for 4-5 minutes. Turn the steaks, spread the other sides with the mustard, then press on the breadcrumb mixture. Grill for a further 4-5 minutes, until the breadcrumb crust is golden brown and the lamb is just cooked through.
To serve, divide the roasted veg between 4 plates and top with the lamb steaks.
Make a double batch of the breadcrumb topping - you can freeze the rest for up to three months.
Always defrost and heat thoroughly before eating. Never re-freeze food that has already been frozen; this includes raw and cooked ingredients such as meat, fish and poultry.
For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home.