Preheat the oven to 425 degrees F and line a rimmed baking sheet with foil. Combine the cranberry juice, jelly and mustard in a medium skillet over medium-high heat; cook, stirring occasionally, until thick enough to coat the back of a spoon, about 15 minutes. Remove 3 tablespoons of the glaze and set aside for brushing. Stir the cranberries into the remaining glaze; set aside for drizzling.
Meanwhile, rub the pork with the olive oil, 1/2 teaspoon salt and a few grinds of pepper. Place on the prepared baking sheet and roast, turning once, 15 minutes. Remove the pork from the oven and brush all over with the reserved 3 tablespoons glaze. Continue to roast until a thermometer inserted into the thickest part of the meat registers 145 degrees F, 5 to 10 more minutes. Let rest 5 minutes, then slice and drizzle with the cranberry glaze.