Rinse pork and pat dry; roll in pepper to coat evenly.
Set an 11- to 12-inch frying pan with ovenproof handle over high heat. When pan is hot, add 1 tablespoon oil and swirl to coat bottom. Add pork (cut tenderloins crosswise to fit pan if necessary) and turn as needed to brown on all sides, 5 to 7 minutes total.
Set pan in a 400° regular or convection oven; bake until a thermometer inserted in center of thickest part of meat reaches 155°, 20 to 30 minutes. Remove from oven and let meat stand in a warm place about 5 minutes.
Meanwhile, in a 1- to 1 1/2-quart pan over high heat, stir onion in remaining 1 teaspoon oil until limp, about 5 minutes. Add cranberries, vinegar, broth, and sugar; stir occasionally until liquid is reduced to about 2 tablespoons, about 7 minutes. Reduce heat to low. Add butter and stir until melted and incorporated into sauce.
Cut meat on the diagonal across the grain into 1/4-inch-thick slices; arrange on 8 plates. Spoon sauce equally over and around meat. Add salt to taste.