Heat your oven to 200°C/Fan 180°C/Gas 6. Drizzle 1 tbsp olive oil into a roasting tin and put it in the oven to heat up.
Peel and crush or grate the garlic. Finely chop the rosemary leaves. Put both in a dish with ½ tbsp oil. Season with salt and pepper. Add the lamb leg steaks. Turn to coat them in the garlicky oil, then leave to marinate.
Scrub the potatoes and chop into bite-sized pieces. Add them to the hot roasting tin. Toss a few times to coat in the oil. Roast in the oven for 10 mins.
While the potatoes roast, scrub and quarter the carrots. After 10 mins, add the carrots to the roasting tin with the potatoes. Toss together and roast for a further 25 mins.
While the veg roast, trim 1cm off the ends of the purple sprouting broccoli spears. After 25 mins, add the broccoli to the roasting tin. Toss everything together. Roast for 15 mins, so the potatoes are golden and tender, and the broccoli and carrots are a little charred.
After you've added the broccoli to the roasting tin, heat a griddle or frying pan. When it’s smoking hot, add the lamb leg steaks. Fry for 4-5 mins, then turn over and fry for 4-5 mins more (this is for pinkish meat – if you prefer well-done, cook the steaks for another 2 mins more on each side). Pop the lamb steaks onto a warm plate. Loosely cover with foil and leave to rest.
Turn the heat down under the frying pan. Make sure any oil, garlic and rosemary from the lamb dish is scraped into the frying pan. Stir in 1 tbsp cornflour. Crumble in the stock cube. Whisk in 300ml boiling water. Gently boil, stirring now and then, for 5-8 mins till thickened. Taste the gravy and season.
Arrange the lamb leg steaks on 2 warm plates. Pour any resting juices into the gravy. Serve the lamb with the veg and the gravy.