Start by frying the onions and garlic with the olive oil for 5-6 minutes until soft. Add in the tuna and spinach leaves and cook for 2 minutes to wilt. Mix in the black olives, crumble in the cheese, pine nuts and season well with salt and pepper to taste. Mix together well and put aside. Next, roll out the pastry to a large rectangle. Cut slits on both sides, 3cm apart from each other and a quarter of the way in from both sides. Spoon the tuna filling into the middle and brush the strips with beaten egg. Stretch the strips over the filling, each side alternatively, creating a tight braid and forming a strudel. Carefully place the strudel onto a large baking sheet, brush all over with beaten egg and sprinkle over the sesame seeds. Bake in the oven at 200`c for 40-45 minutes or until cooked through. Serve thickly sliced with a salad on the side.