For the noodle dough2 cups unbleached all-purpose flour salt, to taste2 large egg yolks1 tablespoon vegetable oil7 -8 tablespoons water1 large egg white, lightly beaten4 tablespoons unsalted butterFor the Potato Filling. 6 tablespoons unsalted butter1 medium onion, finely chopped3 large potatoes, peeled, boiled, and mashed1⁄4 farmer cheese2 ounces colby cheese, grated salt and pepper, to taste