Cut pork lengthways being careful to cut only 2/3 way through. Place between two layers Saran wrap and pound until pork is thin and flat. Season with salt and pepper and line the inside of the pork with fresh spinach leaves.
Heat oil in a pan over medium heat and sauté onions until tender (about 5 minutes). Add diced apple, nuts, sherry, thyme and half of the pitted dates. Heat until apples are tender but not overcooked, sherry has evaporated and the mixture is a just dry (3-4 minutes). Remove from heat and add the remaining chopped dates.
Line pork with spinach leaves. Place half of date and nut mix on closest third of pork tenderloin. Lay slices of prosciutto side by side on a fresh sheet of Saran wrap. Roll up the pork and place seam side down in the center of the prosciutto. Wrap the prosciutto around the pork roll overlapping the ends and tie with string.
Place pork on rack over roasting pan and bake for 30-40 minutes depending on thickness. Remove from the oven, cover with foil and rest for 10 minutes.
Remove pits from whole dates and slice. Gently reheat remaining date and nut mixture.
Slice pork loin into 12 portions. Place a little date and nut mixture on a plate, top with 2 slices of pork and sliced reserved dates.