Boil the potatoes, then cut into small pieces. Peel the onions, cut in half lengthwise and slice into fine half-circles. Heat 6 tablespoons of oil in a skillet over medium heat. Add onions and fry, stirring, separating the rings until the onions are brownish, about 7-8 minutes; they should not get crisp. Add the fennel and cumin and saute another 7-8 minutes on a lower flame. The onions should look a rich reddish-brown now. Add the diced boiled potatoes to the onion mixture and continue cooking. As you saute, mash the potatoes with the back of a slotted spoon or potato masher. Add the garam masala, 2 1/2 teaspoons salt, lemon juice and cayenne to taste. Mix and set aside.
For the cabbage: Cut off the hard stem, remove the hard outer leaves and wash. Use the microwave to soften the cabbage leaves. When soft, spread out one leaf at a time. Snip out the hard core of the outer leaves with a pair of kitchen shears, making a narrow V. Place one tablespoon of stuffing in the center and fold the edges over. Squeeze out the extra moisture; keep leaves tightly closed.
In a 10-inch skillet, heat 4 tablespoons of oil over medium heat. Saute the stuffed cabbage a few pieces at a time until each piece is browned on all sides. Take care not to let the leaves open. When all the pieces are done, lower the heat, arrange the stuffed cabbage pieces in the skillet in tight layers, add 2 tablespoons of water, cover and cook on very low flame for 10 to 15 minutes. Serves 6-8.