Preheat an oven to 350 degrees F (175 degrees C). Grease a baking dish large enough to hold the tenderloin without folding it.
Heat the olive oil in a large skillet over medium-high heat, and brown the pork tenderloin on all sides. Remove the browned tenderloin to the prepared baking dish. Sprinkle all sides of the meat with salt, pepper, and parsley.
Cook and stir the onion and apple in the same skillet over medium heat until the onion becomes soft, about 5 minutes, and pour in the Riesling wine. Scrape all the browned flavor bits off the bottom of the skillet and stir to help dissolve them into the wine. Bring to a boil, and pour the onion, apple, and wine mixture over the tenderloin.
Mix together the apple jelly and balsamic vinegar in a bowl until the mixture is smooth and without lumps. Spread the jelly mixture all over the pork.
Bake loin in the preheated oven until an instant-read thermometer inserted into the center reads 145 degrees F (63 degrees C), 30 to 45 minutes. Allow the tenderloin to rest for 10 minutes before slicing, and serve each slice with a spoonful of the apple-onion mixture.