Heat oven to 400° F.
Cook the couscous according to the package directions.
In a small saucepan, bring the prunes, cider, vinegar, and thyme to a boil. Lower heat to medium-low and simmer 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 an oven-safe baking dish, place in oven, and roast until no trace of pink remains, about 10 minutes.
Return 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 and serve with the couscous.
Tip: To prevent prunes and other sticky foods from clinging to your knife as you chop, moisten a paper towel with vegetable oil and wipe the blade frequently with it.