Make the dough:  combine the flour and salt in the bowl of a standing mixer fitted with a dough hook and process to blend. Add the eggs and sour cream and mix on low until a rough dough comes together. Increase the speed to medium-low and continue to knead until a smooth, pliable, pillowy dough comes together that is slightly tacky, about 3 minutes. Remove the dough from the mixer and lightly flour if excessively sticky. Form into a smooth ball, wrap in plastic wrap, and refrigerate for at least 30 minutes and up to 1 hour. 

While the dough is resting, make the filling. Heat the butter in a small skillet over medium heat and sauté the onion until golden and translucent, 4 to 6 minutes.  Place the mashed potatoes in a medium mixing bowl and add the onion and butter, along with the cream cheese and green onion tops. Stir in the crawfish and season the mixture to taste with Creole Seasoning and Old Bay. One or the other is fine if you do not have both. The filling should be very well seasoned since it is to be encased in a minimally seasoned dough. Refrigerate the filling until you are ready to fill and cook the pierogi. 

When the dough has rested, cut it into quarters and, working on a lightly floured surface, roll the dough to a thickness of about 1/8-inch. Using a 3-inch biscuit cutter or the rim of a glass, cut the dough into as many circles as possible. Brush the dough to remove any excess flour. Place 2 to 3 teaspoons of the filling in the center of each circle. Bring the edges together and press to seal in a half moon shape. If your dough is not sticking to itself, as might happen if it’s too floury from rolling it out, you may need to dampen the edges with a bit of water before pressing them together. Once the edges are pressed to seal, you can seal further by pressing the edges with the tines of a fork, or you can pinch and twist the edges with your fingertips to form a more decorative edging. There are several methods of crimping pierogi and many awesome tutorials are available for this online. Place the filled and sealed pierogi on parchment-lined baking sheets and refrigerate while you finish rolling and forming the remaining pierogi. 

Once you’ve finished forming all of your pierogi, bring a large pot of salted water to a boil. Cook the pierogi, in batches, until they puff and float on the top of the water, 2 to 3 minutes. Remove them using a slotted spoon or strainer and place in a warm bowl while you cook the remaining pierogi. 

Heat a large nonstick skillet over medium-high heat and melt 3 tablespoons of the butter. Sauté about ¼ of the pierogi until crisp and golden on both sides, 2 to 3 minutes per side. Transfer to a serving platter while you saute the remaining pierogi, wiping the pan between batches if necessary and adding an additional 3 tablespoons of fresh butter for each batch. When you have sauteed all of the pierogi, wipe the pan one last time and add the remaining 1 ½ sticks of butter. Cook the butter until it is nutty and golden brown, about 3 minutes. Add a generous squeeze of lemon juice, to taste, and stir in the tarragon. Pour the browned tarragon butter over the pierogi and serve hot.