In a mixing bowl, combine the pumpkin, condensed milk, egg, 3/4 teaspoon cinnamon, ginger, nutmeg and salt. Pour batter into the pie crust.
Bake at 425 degrees F (220 degrees C) for 15 minutes. Reduce oven temperature to 350 degrees F (175 degrees C).
In a small bowl, combine brown sugar, flour, and remaining 1/2 teaspoon cinnamon. Cut in the butter or margarine until the mixture is crumbly. Stir in walnuts. Sprinkle mixture evenly over the pie. Bake pie for 40 minutes or until a knife inserted one inch from the edge comes out clean. Cool and garnish as desired.