Put in a frying pan and heat the margarine and vegetable oil.
Add onions that have been sliced thin and cook until golden brown.
Add mushrooms that have been washed but not peeled or broken.
Cook for 3 minutes.
Remove from pan.
Cut up and roll in seasoned flour the steak and lamb kidneys.
Brown in pan, using a little more oil if necessary.
Combine the onions and mushrooms with the meat in a heavy casserole dish.
Combine together with a bouquet garni or a sprinkle of herbs and add enough hot beef stock to comfortably cover them.
Cook gently on top of stove or in a 300 degree oven until the meat is tender.
Add the oysters that have been drained and the 1 tbs. corn flour that has been mixed with the cup of water or milk.
Reheat.
To avoid a pastry crust which is overcooked, cook it separately and crown the pie with it five minutes before you take it out of the oven.