Use a mandolin for super-thin slices of potato and parsnip.
For added flavour, gently stir-fry chopped silverbeet and radicchio in a little olive oil and butter with a clove of crushed garlic.
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To make gratin, preheat oven to 180C and grease and line a 20cm square cake pan.
Heat oil and butter in a medium saucepan over medium heat. Add onion and thyme and cook until soft. Stir in cream and stock, then simmer until thickened. Stir in half the cheese and season with salt and pepper.
Arrange a layer of potato at the bottom of prepared pan. Drizzle with cream mixture and season with salt and pepper. Top with a layer of parsnip. Drizzle with cream mixture. Season with salt and pepper. Repeat layering with remaining vegetables and sauce, finishing with a layer of potatoes and sauce. Place a piece of foil on top of gratin and bake for 30 minutes. Remove foil, sprinkle with remaining cheese and bake for a further 20 minutes or until golden.
Meanwhile, to make mushroom ragu, heat half the oil in a stockpot over medium heat. Add vegetables and cook until lightly browned. Add wine and port and cook for 5 minutes or until reduced by 2/3. Stir in peppercorns, tomatoes and stock. Bring to the boil, then reduce heat and cook for 15 minutes or until reduced by half. Strain and discard solids.
Heat butter and remaining oil in a medium frying pan over high heat and cook until foaming. Add mushrooms and cook for 3-4 minutes or until lightly browned and all liquid has evaporated. Season with salt and pepper, then add strained beef jus. Simmer for 8-10 minutes or until reduced by half.
Brush steaks with oil, season with salt and pepper and sprinkle with rosemary. Cook on a preheated, oiled barbecue or grill plate over medium-high heat for 4 minutes on each side or until cooked to your liking. Set aside and rest, loosely covered, for 5 minutes.
Serve steak with sauce, gratin, silverbeet and radicchio.