In a large bowl, whisk together the flour and salt.

Cut in the cold butter and shortening using a pastry blender or your fingers until the mixture resembles coarse crumbs. Keep the butter cold.

Gradually add the ice water, one tablespoon at a time, mixing until the dough just comes together. Be careful not to overmix.

Form the dough into a disc, wrap it in plastic wrap, and refrigerate for at least 30 minutes.

On a lightly floured surface, roll out the dough to a 12-inch circle.

Carefully transfer the dough to a 9-inch pie plate. Trim and crimp the edges.

Dock the bottom of the crust with a fork to prevent it from puffing up during baking.

Pre-bake the crust: Line the crust with parchment paper and fill with pie weights or dried beans. Bake in a preheated oven at 375°F (190°C) for 15 minutes. Remove the parchment paper and weights, and bake for another 5-10 minutes, or until lightly golden brown. Let cool completely.

In a medium saucepan, combine the heavy cream, whole milk, granulated sugar, brown sugar, and salt. If using a vanilla bean, add the scraped seeds and the pod to the saucepan. Heat over medium heat, stirring occasionally, until the sugar is dissolved and the mixture is just simmering. Remove from heat and remove the vanilla bean pod.

In a separate bowl, whisk together the egg yolks and eggs until light and slightly thickened.

Slowly temper the egg mixture by drizzling a small amount of the hot cream mixture into the eggs while whisking constantly. This prevents the eggs from scrambling. Continue adding the hot cream mixture gradually until the egg mixture is warmed.

Pour the tempered egg mixture back into the saucepan with the remaining cream mixture. Stir in the ground cinnamon, nutmeg (if using), and vanilla extract (if not using a vanilla bean).

Cook over low heat, stirring constantly with a wooden spoon, until the custard thickens enough to coat the back of the spoon. This should take about 5-10 minutes. Be patient and keep stirring to prevent scorching. The custard is ready when you can run your finger across the back of the spoon and the line holds.

Remove from heat and strain the custard through a fine-mesh sieve into a bowl. This will remove any lumps and ensure a silky-smooth texture.

Pour the custard filling into the pre-baked pie crust.

Bake in a preheated oven at 325°F (160°C) for 30-40 minutes, or until the filling is set but still slightly jiggly in the center. The edges should be set, but the center should have a slight wobble.

Remove the pie from the oven and let it cool completely on a wire rack. As the pie cools, the filling will continue to set.

Once cooled, refrigerate the pie for at least 4 hours, or preferably overnight, to allow the flavors to meld and the filling to fully set.