Preheat the oven to 400 degrees F. Sprinkle the pork with the pumpkin pie spice, 1/2 teaspoon salt, and pepper to taste. Heat the olive oil in a large ovenproof skillet over medium-high heat. Add the pork and cook, turning, until browned on all sides, about 6 minutes.
Halve the apples and scoop out the cores. Cut 1/2 tablespoon butter into 4 pieces; put a piece in each apple half. Sprinkle with half of the shallot and a pinch of salt. Add the apples to the skillet with the pork, cut-side up, and transfer to the oven. Roast until a meat thermometer registers 145 degrees F, about 25 minutes.
Meanwhile, combine 1 3/4 cups water, the remaining shallot and a pinch of salt in a saucepan; bring to a boil. Add the barley, cover and cook over low heat until the water is absorbed, 12 minutes. Add the parsley.
Transfer the pork and apples to a cutting board. Return the skillet to medium-high heat and add the cider, vinegar and 1/4 cup water; simmer, stirring, 2 minutes. Stir in the remaining 1 tablespoon butter. Slice the pork and serve with the pan sauce, apples and barley.