Preheat oven to 200ºC (390ºF). Place the garlic, thyme, oil, salt and pepper in a bowl and mix to combine. Brush the mixture over the lamb, secure with kitchen string and place in a baking dish. Roast for 15 minutes, add the tomatoes and leek, drizzle with the extra oil and roast for a further 15 minutes for medium or until cooked to your liking. Slice and serve with the tomatoes and leek and drizzle with balsamic glaze to serve.