Preheat the oven to 180C/350F/Gas 4.
For the lamb with mustard crust, rub the lamb steaks with olive oil, season well with salt and freshly ground black pepper and place into a hot frying pan to fry for three minutes on each side.
Spread the mustard over the lamb.
Place the breadcrumbs and herbs into a food processor and blend together. Press the breadcrumb mixture onto the lamb steaks and set aside.
For the sweet potato fondant, put the sweet potato rounds into a small frying pan with the oil and hot water. Season well with salt and freshly ground black pepper and then cover the pan with foil and cook for two minutes, turning the sweet potato once.
Remove the foil cover and transfer to the oven and bake for 12 minutes, until cooked through.
To serve, place the coated lamb steaks onto a plate and serve the sweet potato alongside.