Make incisions in the pork every 2 to 3 inches, and insert garlic slices. In a small bowl, combine salt, pepper, sugar, and marjoram, and coat the pork evenly with the mixture. Cover with plastic wrap, and let stand at room temperature for 1 hour.
Preheat oven to 375 degrees. Sprinkle flour over the pork to lightly coat. Shake to remove any excess flour. In a large skillet, heat oil over medium-high heat. Add pork, and brown on all sides. Transfer pork to a large roasting pan. Grate onions over the pork. Add 1 1/2 cups water to roasting pan, and roast, covered, for 1 hour, basting occasionally.
In a medium bowl, soak prunes in enough cold water to cover, for 15 minutes. Drain, and add to roasting pan. If necessary, add a little water to roasting pan to prevent pork loin from sticking. Cover, reduce heat to 325 degrees, and roast until temperature of pork loin reaches 140 degrees on an instant-read thermometer, 15 to 30 minutes.
Remove pork loin from roasting pan, and let rest for 10 minutes. Place roasting pan on stove, heat over medium-high heat, and cook pan juices until reduced and thickened enough to coat the back of a wooden spoon.
Slice pork, arrange on a serving platter, and serve with sauce.