Preheat oven to 350°F. Place walnuts and oats on a sheet tray. Bake 4 to 5 minutes, or until very lightly browned and toasted.
Add toasted mixture to the bowl of a food processor, fitted with a metal blade. Pulse until finely ground. Add flour, sugar, melted butter, salt and cinnamon to bowl. Pulse again to incorporate everything. If the mixture looks too wet add another tablespoon of flour. Continue until mixture looks like wet sand.
Transfer and evenly press mixture into bottom and 1/3 up the sides of a large 9-inch spring form pan (or 13x9” brownie pan). Bake for 15 minutes. Let cool completely.
Mix or whisk all ingredients in a large bowl until well combined and smooth. Pour into pre-baked crust pan.
Bake for 50-60 minutes or until filling is set and a knife comes out clean. NOTE: if using a spring form pan be sure to cover the bottom and up the sides (about 1/3 of the way) with aluminum foil to catch the drips out the bottom. It WILL leak.
Cool to room temperature. Serve with fresh whipped cream sweetened with maple syrup and decorate with lightly toasted pecan halves, if you like.
Note: for regular pie pastry , bake filling in raw dough, on bottom oven rack at 400°F for 15 minutes then lower to 350°F for 30 minutes.
Note again: if using frozen store-bought pie shells (i.e. Tenderflake) thaw first before pouring in filling to bake. VERY IMPORTANT.