Heat butter in an oven-proof skillet over medium heat. Place pork tenderloin in butter, and brown on both sides. Remove from skillet, and set aside.
Stir in onions, and cook 2 to 3 minutes. Stir in apple, and cook until golden brown. Stir in flour; cook about 30 seconds.
In a bowl, stir together stock and apple cider. Stir into skillet, and bring to a boil. Return tenderloin to skillet. Season to taste with salt and pepper. Cover skillet.
Bake in preheated oven until tender, about 45 minutes. Remove tenderloin to a cutting board, and cut into 4cm slices.
Pour the sauce, including the onions and apples, into a blender or food processor and pulse until smooth. Return the sauce to the pan, and reheat over medium-low; stir in 2 tablespoons cream, and heat until warm. Serve sauce spooned over sliced tenderloin.