Secure the steaks with short skewers to form neat rounds. Heat the oil in a wide, heavy frying pan over high heat and brown the steak for about 5 minutes. Transfer the meat to a large fireproof casserole. Grind the saffron with the sea salt using a pestle and mortar. Bring the stock to the boil, dissolve the saffron and salt in the stock and pour it over the meat in the casserole. Add the pumpkin and the oregano sprigs to the casserole and simmer, partly covered, over very low heat until the meat is tender -- about 1 ¾ hours. From time to time, remove any scum or fat which has risen to the surface
Lift the steaks out of the casserole and remove the skewers, keep the steaks warm while preparing the sauce. Discard the oregano. Puree the pumpkin with a little of the stock in a blender or food processor, then blend in the remaining stock. Reheat the pumpkin puree and season it with the 1/2 teaspoon of salt and the pepper. Spoon it on to four warm plates and place a steak in the centre of each. Drop small spoonfuls of yoghurt into the puree surrounding the meat and draw a pattern. Garnish with the walnuts and oregano leaves.
