1. Preheat oven to 350ºF.
2. Melt butter in small saucepan.
3. Add flour all at once. Cook, stirring constantly, until mixture is golden brown.
4. Add syrup and water and cook, whisking constantly until thickened.
5. Allow to cool for 10 minutes. Add nuts.
6. Pour warm filling into prepared pie shell.
7. Bake in a preheated oven for 35 to 40 minutes or until filling is set and crust is golden brown.
8. Cool on wire rack.
9. May be decorated with nuts, pastry maple leaves, etc. Best if garnished with whipped cream.
Note: You can substitute ½ cup water for the fresh mape sap if its not available.