Pastry: In large bowl, whisk flour with salt. Using pastry blender, cut in butter and lard until in fine crumbs with a few larger pieces.
In liquid measure, whisk egg yolk with vinegar; add enough ice water to make 1/3 cup. Drizzle over flour mixture, tossing with fork and adding a little more water if necessary to form ragged dough. Press into disc; wrap in plastic wrap and refrigerate until chilled, about 30 minutes. (Make-ahead: Refrigerate for up to 24 hours.)
On lightly floured surface, roll out pastry to 1/8-inch thickness; fit into 9-inch pie plate. Trim to leave 1/2-inch overhang; fold overhang under and flute edge. Sprinkle with walnuts, if using.
In large bowl, whisk eggs with brown sugar until creamy; whisk in maple syrup, cream and flour until smooth. Pour into pie shell.
Bake on foil-lined baking sheet in bottom third of 375°F oven until pastry is golden and filling is puffed and dry to the touch but still trembles, 40 to 45 minutes. Let cool on rack.