Rub the pork with a mixture of mustard, soy sauce, beer, and pepper. Set aside. Soak the prunes in ½ cup of water and an equal amount of vinegar, along with the cloves. Set aside.
In a baking dish, place the pork loin with the prunes, without the soaking liquid. Add an onion cut into four parts and gradually add the liquid. Bake at medium heat for about an hour and a half.
