Heat a large Dutch oven over medium heat. When the pan is hot, add 2 tablespoons of butter. Season the pork generously with salt and pepper, to taste, and sear on all sides in the hot pan until golden brown; set aside. Add the onions and apples and season again with salt and pepper. Cook until they begin to caramelize, then pour in the hard cider and scrape the brown bits off the bottom of the pan. Bring to a simmer and nestle the browned pork roast back into the pan. Cover with a lid and put in the oven to braise until the internal temperature reaches 145 degrees F on an instant-read thermometer, 25 to 30 minutes.
Remove the pork roast from the pot and transfer to a carving board; tent with foil to keep warm. Transfer the contents of the pot to a food processor or blender and puree, then return the puree to the pan. Bring to a boil, season with salt and pepper, to taste, then reduce the heat to low and add more cider if gravy is too thick. Add the remaining tablespoon of cold butter, whisking constantly as it melts. Remove from heat when the gravy is smooth and shiny and the butter is completely melted.
Slice pork loin roast and arrange on a serving platter. Serve with sauce and enjoy!