Butterfly the steak by cutting it lengthways. In a frying pan, melt butter over medium heat then saute mushrooms for about a minute. Stir in oysters and garlic, cook for about 1 minute.
Stir in lemon juice, brandy, garam masala and peppercorns. Cook for a few minutes then remove from heat. Slowly stir in enough breadcrumbs to make a stuffing.
Fill the steak with stuffing then pin it closed with wooden toothpicks. Heat oil and butter in a frying pan then brown the steak on both sides. Place in a baking dish and roast in oven until done, about 40 minutes.