Grease and line a 24cm square tin. Whisk together the yogurt, lemon zest and juice and set aside.
Bring the milk and rosemary to a simmer over a medium heat. Whisk in the polenta, reduce the heat and cook, stirring constantly, for 5 minutes. Add the parmesan and butter, season and tip into the prepared tin. Chill for 1 hour.
Turn the polenta out onto a chopping board and slice into 2cm-thick chips.
Set a griddle pan over a high heat. Season the steaks, brush with oil and cook for 2-3 minutes on each side or until done to your liking. Leave to rest for 5 minutes before slicing.
Meanwhile, set a frying pan with the oil over a medium heat. Cook the chips for 2-3 minutes on each side, until crisp.
Serve the steak, chips and leaves with the lemon yogurt dressing on the side.