Heat your oven to 200°C/Fan 180°C/Gas 6.
Trim the carrots and cut in half lengthways. Trim the leeks and cut them into thirds widthways. Put them all onto a baking tray. Drizzle with ½ tbsp. oil and season. Slide the veg into the oven and cook for 20 mins.
Peel and grate the garlic, then pop into a bowl. Add the cumin seeds and 1 tbsp oil. Season and mix well. Rub this mix all over the lamb leg steaks.
After the veg has cooked for 20 mins, place the lamb leg steaks on top of the veg and cook for 15 mins till the lamb is cooked through. If the lamb is thick, it may take slightly longer. Cut halfway through a steak and see if it’s cooked to your liking. If not, cook them for a little longer.
While the veg is cooking, peel and chop the parsnips into small cubes. Place in a pan and cover with boiling water. Simmer for 10 mins till soft. Drain.
Put the parsnips back into the pan and onto the heat. Drain and rinse the butter beans. Add to the cooked parsnips. Cook for 5 mins to warm the beans through and dry off the parsnips.
Season the parsnips and beans. Add ½ tbsp oil and mash them. Use a food processor if you prefer a smoother mash. Roughly chop the mint leaves.
Serve the cumin lamb with the roasted veg and white bean mash. Scatter over the chopped mint.