Place butter in a 10-in. ovenproof skillet. Place in a 425° oven just until melted. In a large bowl, beat the flour, eggs, milk and salt until smooth. Leaving 1 tablespoon melted butter in the skillet, stir the remaining butter into the batter. , Pour batter into hot skillet. Bake for 15-20 minutes or until edges are golden brown., Meanwhile, in a small saucepan, warm pie filling over low heat until heated through. Pour into center of puff pancake. Sprinkle with walnuts if desired. Serve immediately.