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 in the garlicky oil and 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. Toss together and roast for a further 25 mins.
While the veg roast, trim 1 cm off the ends of the purple sprouting broccoli spears. After 25 mins, add the broccoli to the roasting tin. Toss everything together, Roast for 10-15 mins, so the potatoes are golden and tender, and the broccoli and carrots are a little charred.
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 more mins (this is for pinkish meat. If you prefer well done, cook for another 2 mins more on each side). Pop the lamb steaks on 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 are scraped into the frying pan. Stir in 1 tbsp of cornflour. Crumble in the stock cube. Whisk in 300ml boiling water. Gently boil, stirring now and then, for 5-8mins 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.