Sift together the flour, sugar, salt, and baking powder in a bowl. Mix the milk and oil together in a separate bowl. Make a 'well' in the center of the dry ingredients and pour in the liquid ingredients. Slowly mix together the dough by working from the center and incorporating the flour from the edges of the well until you have a smooth, soft dough. Knead the dough for 8 to 10 minutes, adding a little flour if the dough is too sticky. Place the dough in an oiled bowl, cover with a damp tea-towel or kitchen towel, and leave in a warm place for at least 1 hour, or until the dough has doubled in size. Then knock back or punch down the dough, and then divide and form the dough into five equal-sized balls. For the filling: Pulse together the pistachios, raisins, and sugar in a food processor until the mixture forms a coarse powder. Divide the mixture into 5 equal portions. Preheat the grill to its highest heat setting. Place a heavy baking sheet on the top shelf to heat. On a lightly-floured work surface, using a lightly-floured rolling pin, roll out each of the 5 portions of dough balls into thick circles. Fill half of each circle with one portion of the filling, leaving about a 1-inch margin or border around the edge. Wet the dough around the edges with a little water, and fold each circle in half to enclose the filling. Pinch the dough around the edges to close. For the topping: Gently roll out each naan into a teardrop or oval shape. Sprinkle the flaked almonds over the naan. Place the naan on the hot baking sheet and grill for about 1 to 2 minutes, or until there are nice brown spots on the surface. Brush the naan with the melted butter and serve hot.