Before you start, preheat BBQ hot plate or grill to medium. Preheat oven to 220°C (or 200°C fan bake).
Pat lamb dry, rub with oil and season with salt and pepper. Cook on BBQ grill for 20-25 minutes for medium (depending on thickness), or until cooked to your liking. Turn every 5 minutes to ensure even cooking. Rest, covered. Alternatively, heat oil in a large fry-pan on medium-high heat and cook lamb, fat side down for 3 minutes. Flip and cook a further 2 minutes, then transfer to a lined oven tray and cook for 10-14 minutes for medium, or until cooked to your liking.
Prep potatoes and add to a large pot. Cover with hot tap water. Cook on high heat for 20-25 minutes with the lid on, until tender and cooked through. Drain well, return to pot along with butter and season with salt and pepper.
Prep tomatoes and capsicum and place on a platter. Crumble over feta and drizzle with balsamic vinegar and olive oil. Season with salt and pepper.
To finish, slice lamb thickly against the grain.
Serve lamb with potatoes and salad.