Pour the gravy into an ice cube tray (you should have 12 cubes) and allow to freeze.
Boil peeled and roughly chopped potatoes in a pot of salted water until tender.
Strain potatoes and pass through a mouli or mash with a potato masher in a medium bowl.
Stir through butter & rosemary and season to taste with salt.
Divide the mashed potato into 12 equal portions.
Set up a crumbing station: place flour into a small bowl, whisk together eggs and milk into a small bowl and place breadcrumbs in a medium bowl.
Heat a deep fryer to 180C.
Place a portion of potato on the palm of your hand, press a frozen gravy cube into the potato and encase the gravy with the potato, forming a ball. Place on a tray and store in the freezer while you form the remaining balls.
To crumb, place a ball into the flour, shake off excess, place into the egg mix, rotating to coat, strain off excess egg mix, place into the breadcrumbs and roll to coat.
To double coat the croquettes, return to the egg mix, roll to coat, return to the breadcrumbs and roll to coat. This ensures the croquettes will not split when fried and provides an extra crispy exterior.
Fry croquettes in batches until golden, about 8 minutes.
Serve topped with grated parmesan.
Croquettes can be made ahead of time, stored in the freezer. To cook, lower fryer temperature to 160C and cook for 15 minutes.