Prep the onions: Trim off the ends and, sliced lengthwise to the center (as if you are slicing it in half lengthwise (vertically, like the Prime Meridian), but only go halfway through). Remove the skin.
Fill a large pot halfway with water and bring to a boil. Using a slotted spoon, immerse the trimmed onions into the boiling water. Partially cover pot and gently boil onions for 20 minutes. Using the slotted spoon, transfer onions into a large bowl filled with cold water. Pour out heated water and refresh with more cold water. Let onions cool in the water for a few minutes. To keep from burning your hands, work under water to gently separate the softened outer layers of each onion. Put onions layers in a strainer to dry and set aside.
Preheat the oven to 350F.
Make the stuffing: Put the beans into a food processor and pulse until finely chopped. Add the beans, along with the rest cooked rice to a medium sized bowl. Heat 1 tsp oil in a small frying pan. Add chopped garlic and cumin seeds to pan and cook over medium heat, stirring until fragrant, about 1 minute. Turn off heat if garlic starts to turn brown. Scrape cumin and garlic into bowl with rice. Add salt and pepper. Stir rice to combine with spices. Add the feta and the prunes, and blend it all together well, using your hands.
Stuff the onions. Add a heaping scoop of the filling to the center of an onion layer. Roll the onion layer around the beans and put in the baking dish, seam side down. Keep going until all the filling has been used up. Brush the tops of the rolls with some olive oil and sprinkle with some fresh ground pepper.
Cover the baking dish with foil. Bake for about 40 minutes, long enough to ensure it’s all heated through and the cheese has melted. Remove dish from the oven.
Turn the oven up to broil. Remove foil from casserole. Baste onions with additional olive oil. Place under the broiler for an additional 5 minutes to brown them.