To make the dough, dissolve the yeast in the lukewarm milk and stir in ¼ cup of the flour.
By hand: Break the eggs into a large bowl and whisk briefly to blend them. Add the yeast mixture and the remaining 1¼ cups all-purpose flour and stir with a wooden spoon until very smooth. Add the sugar, salt, and butter and mix well. Add the pastry flour ½ cup at a time, mixing well. Turn the dough out onto a lightly floured surface and knead for 10 minutes, or until it is smooth and firm, incorporating additional all-purpose flour only as necesssary.
Using a stand mixer: Break the eggs into the bowl of mixer. Using the whisk attachment, whisk the eggs briefly to blend them. Change to the dough hook. Add the yeast mixture and the remaining 1¼ cups all-purpose flour and mix on low speed until very smooth. Add the sugar, salt, and butter and mix well. Add the pastry flour and knead on low speed for 5 minutes, or until smooth and firm.
Transfer the dough to a clean bowl, cover well with plastic, and let rise in a cool place for 2 hours, or until doubled.
Meanwhile, prepare the filling (or fillings).
For the oniony greens filling: Thoroughly wash the greens and spinach, then chop them into 1-inch lengths, discarding any thick stems; set aside. Heat the bacon grease, butter, or oil in a large heavy skillet or wok over medium heat. Add the garlic and scallions and cook for about 1 minute, until the scallions begin to soften. Raise the heat to high, add the chopped greens and salt, and cook, stirring constantly, until they start to wilt. Cover, lower the heat slightly, and let steam for about 1 minute. Turn out the greens onto a platter to cool to room temperature. Taste for salt, and set aside.
For the greens and sweet potato filling: Place the sweet potatoes in a pot with about 1 inch of cold water and bring to a boil, then cover and simmer Over medium-low heat until just tender. Drain and set aside.
Thoroughly wash the greens, then chop them into about 1-inch lengths, discarding any thick stems. Heat the grease, butter, or oil in a large heavy skillet or wok over medium heat. When it is hot, add the garlic and sauté briefly, then toss in the greens and salt, raise the heat, and cook, stirring constantly, until the greens start to wilt. Cover, lower the heat slightly, and steam for about another minute. Transfer to a platter to cool to room temperature, then toss with the sweet potatoes and black pepper. Taste for salt and set aside.
Place a rack in the center of the oven and preheat the oven to 450°F. Lightly grease a baking sheet (or two, if making two pies) with butter or bacon grease.
Turn the dough out onto a lightly floured surface and cut in half. (If not making two pies now, place half the dough in a plastic bag and refrigerate for up to 24 hours before using, or freeze for up to a month. Thaw overnight in the refrigerator and bring back to room temperature before continuing.)
For each pie, cut the dough into 2 slightly unequal pieces: The larger one will be the bottom crust. Press the cut edge of the larger piece in flour, then flatten it on a lightly floured surface into an oval. Use a rolling pin to roll the dough out into an oval about 16 inches long and 10 inches wide. The dough may spring back a little—if it does, start flattening the smaller piece of dough in the same way, moving from one to the other to give the dough time to relax so you can stretch it farther. Roll the smaller piece of dough into an oval about 15 inches long and 8 inches wide.
Place the larger piece of dough on the prepared baking sheet. Mound the filling on it, leaving a generous 1-inch rim all around uncovered. Place the top over the filling, and pull down on the sides if necessary to stretch it over. Fold the edges of the bottom dough up over the edges of the top to seal in the filling, then twist it, making a twisted rolled edge all around to seal. Use a sharp knife to make about 10 slits in the top crust. Brush with the melted butter.
Bake the pie for 8 minutes, then lower the heat to 400°F and bake for another 8 to 10 minutes (if making two pies, bake one at a time). The pie will be a deep, rich golden brown.
Transfer the pie to a wire rack to cool for 20 minutes.
Serve warm or, our preference, at room temperature. Because the pie is an oval, the easiest way to serve it is in slices cut right across the pie.