Preheat the oven to 350 degrees F (175 degrees C).

Melt butter in an oven-proof skillet over medium heat. Sear tenderloin in melted butter, then transfer to a plate.

Add onions to the skillet; cook and stir for 2 to 3 minutes. Stir in apple and cook until golden brown. Stir in flour and cook for about 30 seconds.

Whisk chicken stock and hard cider together in a bowl; pour into the skillet and bring to a boil while scraping the browned bits of food off the bottom of the pan. Return tenderloin to the skillet; season with salt and pepper.

Cover the skillet, transfer to the preheated oven, and bake until is pork is slightly pink in the center, about 45 minutes. An instant-read thermometer inserted into the center should read at least 145 degrees F (63 degrees C).

Transfer tenderloin to a cutting board, and cut into 1 1/2-inch slices.

Place onions, apples, and all liquid from the skillet into a blender or food processor; pulse until smooth. Return sauce to the pan, and reheat over medium-low. Stir in cream and heat until warm.

Spoon sauce over sliced tenderloin to serve.