In a medium bowl, toss cubed pork with salt; set aside. Heat 1 tablespoon olive oil in a large nonstick skillet over medium-high heat. Add half the pork tenderloin cubes and saute, stirring constantly, until meat is browned on all sides. Remove with slotted spoon into large bowl and repeat with remaining pork. Remove and set aside.
Add onion and mushrooms to empty skillet; saute until vegetables are soft and liquid has evaporated, about 2 to 3 minutes. Remove onion and mushrooms with slotted spoon and add to pork mixture; then add apples, sage, salt, red pepper flakes, and mustard. Mix well.
In a small saucepan over medium heat, melt butter. Stir in flour to form a roux. Slowly whisk in apple juice until mixture is smooth and thickened slightly. Add sauce to pork mixture and stir to combine.
Preheat oven to 350°. Coat a 10-inch deep-dish pie plate with vegetable cooking spray. Place pork mixture in pie plate.
Sprinkle with sugar. Roll out pastry into a 12-inch circle and lay over the filling, creating a decorative edge. Brush entire crust with egg wash and cut several ½-inch slits. Bake 45 minutes, or until crust is golden brown and pie is bubbling.