There is a theory that, since flies are partial to molasses and have to be chased away while the cook is making the pie, this is how the pie was named.

Pastry for a 1-crust pie

Crumb Mixture
3/4 cup flour
1/2 teaspoon cinnamon
1/8 teaspoon each nutmeg, ginger and cloves
1/2 cup firmly packed brown sugar
1/2 teaspoon salt
2 tablespoons shortening

Filling
1/2 tablespoon baking soda
1/2 cup molasses
3/4 cup boiling water
1 egg yolk, beaten well

Line a 9-inch pie pan with pastry. Refrigerate while you prepare the Crumb Mixture and Filling. Combine flour with brown sugar, salt and spices. Pinch in the butter until the mixture looks mealy. set aside.

Stir baking soda, molasses and egg yolk into boiling water and let cool completely. Put one-third of the Crumb Mixture into the unbaked pie shell. Add liquid, then sprinkle top with remaining crumbs and bake at 400 degrees F for about 10 minutes or until crust begins to brown. Turn oven to 325 degrees F and bake until the center is a bit firm, about 30 minutes.