To make the marinade: Combine the curry powder, lemon juice and olive oil in a small bowl and mix well. Brush the marinade over the lamb steaks and set aside for 5-10 minutes to allow the flavours to develop.
To cook the vegetables: Heat the oven to Gas Mark 6, 200°C (400°F). Cut the potatoes in half lengthways and place in a roasting tin. Drizzle with a little oil and season with salt and black pepper. Roast in the preheated oven for 20 minutes. Then toss the broccoli and red onions in a little oil and seasoning. Add them to the roasting tin with the potatoes and roast for another 8-10 minutes. Transfer to a serving dish.
To cook the lamb: Meanwhile heat a griddle pan or barbecue to high. Season the steaks and add them to the pan or barbecue. Allow to cook for 3-4 minutes on each side or until cooked to your liking. Leave to rest for 2 minutes.
To make the dressing: Combine the ingredients, taste and season.
Drizzle a little dressing over the vegetables and serve with the steaks. Serve the remaining dressing on the side.