Peel the potatoes and place in a bowl of cold water to prevent browning. Trim both ends of each potato so they are flat and the potatoes can stand upright, then use a peeler or melon baller to scoop out the center of the potato, leaving a 1/2-inch border. Return the scooped potatoes to the water.
Heat several inches of oil in a large, heavy pot to 300 degrees F. Drain the potatoes well and pat them dry, then fry until they are slightly golden but not completely tender, 10 to 12 minutes, turning occasionally. Remove the potatoes to a paper towel-lined plate to drain, taking care to pour any hot oil back into the pot as you remove them.
Meanwhile heat 2 tablespoons vegetable oil in a large, heavy skillet or pot. Add the onion and cook until translucent, stirring occasionally. Add the ground beef and season with salt, pepper and cinnamon. Cook, stirring to break up any lumps, until the beef is browned. Remove from the heat and stir in the pine nuts.
Stuff each potato with the filling and place upright in a baking dish.
Pour a thin layer of stock into the baking dish and squeeze the lemon over the potatoes. Bake for 15 to 20 minutes until the potatoes are completely tender. Sprinkle with parsley leaves.