The unorthodox system of cooking the filling on top of the stove and then pouring it into a fully baked, crisp pie shell alleviates the tendency for the crust to be damp under the pumpkin filling. It is inspired by a winning recipe from a contest sponsored by the Borden Company in 1931. Apparently cooks are always looking to improve on a classic. The broiled topping, crisp and crunchy over the smooth pumpkin filling is optional, but it elevates this pie to new heights.
Preheat the oven to 425 F. Roll out the dough and fit in into a 9-inch pie pan. Trim and flute the edges, then bake the pie shell fully. Cool completely before filling.
To make the filling, whisk together the evaporated milk and gelatin in a medium, heavy-bottomed saucepan. Let stand for a few minutes to soften the gelatin. Add the cinnamon, ginger, cloves and salt then whisk until blended. Add the eggs and sugar and whisk again until blended and smooth.
Cook over moderate heat, whisking almost constantly, for 7 to 10 minutes, until the mixture thickens slightly and you see wisps of steam rising, but do not let it boil.
Remove from the heat and add the pumpkin, then whisk until completely smooth. Pour the filling into the prepared pie shell and refrigerate for at least 4 hours or overnight if you prefer. The filling will become firm as it cools.
To make the topping, preheat the broiler and position a rack so that the surface of the pie will be about 4 inches from the heat. Combine the walnuts, brown sugar, salt and butter in a small bowl and stir briskly with a fork until evenly mixed. Spread over the surface of the cooled pie.
Broil for about 2 minutes, or until the topping is lightly browned and bubbly. Watch the pie closely during this time, and rotate it once or twice as necessary so the topping browns evenly.
Let cool before serving. The topping may be broiled several hours ahead; although it will lose a little of its crunch, it will still be very good.