Put the pork fillet on a board and using a sharp knife, carefully trim off as much excess fat and sinew as possible. Using a long thin knife, cut a 1 1/2-inch-wide slit through center of pork loin lengthwise. Insert a wooden spoon handle and widen the opening. In a small bowl, mix wilted spinach, prunes, egg yolk, garlic and seasoning. Put themixture into a piping bag and fill .Seal the fillet and complete cooking in a hot oven 180`c for 10-15 minutes. Boil the sweet potato until soften, drain and mix in butter, maple syrup, thyme and seasoning and crush with fork. Fry asparagus in butter and serve with the pork fillet.