Heat oven to 400° F. Cook the couscous according to the package directions.
Meanwhile, in a small saucepan, bring the prunes, cider, vinegar, and thyme to a boil. Simmer on medium-low until reduced by half, about 8 minutes.
Meanwhile, heat the oil in a large skillet over medium-high heat. Season the pork with the salt and pepper. Sear the pork, turning once, until browned but not cooked through, about 1 minute per side.
Transfer to a baking dish, place in oven, and roast until no trace of pink remains, about 10 minutes.
Return the skillet to medium-low heat, add the onion, and cook until softened, about 6 minutes. Add the garlic and cook for 2 minutes more. Add the cider mixture and simmer for 2 minutes more.
Spoon the pan sauce over the pork. Serve with the couscous.