Slit the steaks to make a pouch, stuff with oysters, and wrap with pancetta.
Heat 50g of butter in a heavy frying pan over medium heat and add the steaks.
Cook steaks for 3 minutes per side for medium-rare then remove from pan.
Deglaze the pan with reserved oyster water, scraping the bottom for flavor.
Pour the pan juices over the cooked steaks.
Preheat the oven to 190°C.
Warm olive oil in a heavy casserole dish over medium heat.
Add potatoes, tomatoes, onions, garlic, thyme, and sage, and bring to a boil.
Season well and simmer for 10 minutes, then bake covered for 30 minutes at 190°C.
Cool slightly, stir in olives and parsley, and serve with the steaks.