Combine the all-purpose flour, cake flour, and salt in a large mixing bowl and make a well in the center. Add the eggs and water. Using a large fork or whisk, blend just the liquids together. Switch to a wooden spoon and gradually stir in the flour from the sides of the bowl. When the dough forms a ball, turn it onto a flat surface dusted with just enough flour to keep the dough from sticking. With floured hands, knead the dough for about 7 to 8 minutes. Put the dough in a bowl, then cover the bowl with plastic wrap and set it aside while you make the pierogi filling. Put the potatoes in a stockpot and add enough water to cover them by several inches. Salt the water well and bring it to a boil. Cook the potatoes until tender, about 10 minutes. While the potatoes cook, melt the butter in a medium-size skillet. Add the onion and sauté it over medium heat, stirring often until light golden, about 8 minutes. When the potatoes are done, drain them and then transfer them to a large bowl. Add the sautéed onion, sour cream, Cheddar cheese, 3 tablespoons of the milk, and about 1/4 teaspoon of salt. Mash well by hand, adding enough salt and pepper to ensure that the filling won't be bland. If the potatoes seem a bit dry, mash in another 1 to 2 tablespoons of milk. Set the mashed potatoes aside. Divide the dough in half. Working on a floured surface with 1 half at a time, roll the dough very thin - thinner than pie pastry. If the dough springs back (it should stay where you've rolled it), let it rest briefly. Using a 3 1/2- to 4-inch round cutter (the open end of a clean, empty can works great), cut the dough into rounds. Gather and roll out the scraps to make as many as possible (about 24 to 30). Then put 1 scant tablespoon of the filling in the center of each round, molding the filling into a football shape. For each pierogi, dip your finger into a bowl of water and run it halfway around the perimeter of the dough, moistening a band about 1/2 inch wide. Fold the dry half over the moistened half (the dough will stretch slightly as you fold it) and firmly press together the edges to seal the pierogi. Bring a large pot of salted water to a boil. Carefully slide 5 or 6 pierogi at a time into the boiling water. Once they rise to the surface, let them boil for 7 minutes. Then use a slotted spoon to transfer them first to paper towels to drain for 30 seconds and next to a baking sheet lined with plastic wrap. Now you can use half of the pierogi to finish preparing the recipe and freeze the other half (for up to a month) for a future meal. Melt 2 tablespoons of the butter in a very large, heavy skillet. Add the onions and sauté them over medium heat for 5 minutes, stirring often. Add the remaining butter, the cabbage, and 2 tablespoons of water. Salt lightly. Continue to sauté, partially covered, until the cabbage is wilted and lightly browned, about 7 to 8 minutes. Add 12 to 15 pierogi to the pan. Cover and heat the mixture for 3 to 4 minutes. Serve at once. Makes enough to feed a family of 4.