Season the steaks generously with salt and pepper, then rub with mustard seed oil. Set aside for 30 minutes. Pour the stock into a medium saucepan and boil rapidly until reduced to one cup.
Meanwhile, steam the potatoes for 20 minutes until tender, then crush lightly. Fry in hot (180ºC) vegetable oil, until crisp and golden. Drain well. Combine the rosemary in a mortar with 2 tsp salt and pound well, then toss with the crisp potatoes.
Add the port wine and peppercorns to the stock and simmer gently. Mix the butter and flour, whisk in, then simmer until thickened.
Grill the striploin steaks over hot barbecue coals for 8 minutes, turning regularly, until medium. Allow to rest for 3 minutes. Carve, then serve with the potatoes and sauce.