Preheat oven to 350°F. Salt tenderloin on all sides. Preheat Dutch oven pan on stove so it is quite hot. Heat 1tbsp oil in the pan and brown the tenderloin on all sides, then remove from pan. Fry onion in the same pan with remaining oil. Remove from heat when onions are translucent. Make a mound with the onions and add the minced garlic, then place the tenderloin on top. Place prunes and potatoes around, and add the white wine and thyme. Sprinkle salt on the potatoes. Cover and bake 1 hr (45min for smaller tenderloin).