The night before, you need to place the meat in a large bowl along with the bay leaf and thyme, then pour the porter and port all over it. Put a plate on top to keep the meat pushed down and leave in a cool place overnight.Next day, when you are ready to cook the meat, pre-heat the oven to gas mark 1, 275°F (140°C). Then melt half the butter and oil in the casserole and heat gently. Drain the meat (reserving the liquid and herbs) and pat the steaks dry with kitchen paper. Now turn the heat to high and brown the steaks (in two batches) to a rich brown on both sides. Now add the rest of the butter and oil to the casserole. As soon as it begins to foam, add the onion and brown this for about 8 minutes before adding the garlic and frying for another 2 minutes.Now return all the meat into the casserole to join the onions. Stir in the flour to soak up the juices, then pour in the marinade (including the bay leaf and thyme), add the walnuts and season well. As soon as it reaches a gentle simmer, put a lid on, then transfer the casserole to the middle shelf of the oven and forget all about it for 3 hours, by which time the meat will be tender and the sauce marvellously dark and rich.