Cook's Illustrated's Polish Dumplings

These are fun, and different than our usual fare.  These pierogis filled with meat are delicious, and I like that the dough isn't a pastry dough but more like a pasta dough, and they make for a tasty dumpling.  I gave these an 8, and the boys all gave these a 9.  I felt like they could've used more flavor, but regardless, they are fun and good.  The calories for four of the dumplings is 372, but that includes extra carmelized onions on the top and we didn't have that.  Anyway, here's what to do:

2 c flour, plus more for dusting the work surface
1/2 t salt
1 large egg
1 large egg yolk
1 T vegetable oil
4-6 T cold water
4 T unsalted butter

4 T unsalted butter
3 large onions halved and sliced thin
salt

Meat or cheese fillings

Pulse the flour and salt together in a food processor.  With the machine running, slowly add the eggs and oil, about 30 seconds.  Slowly add the water until the dough forms a ball.  Knead the dough for about 2 minutes by hand, cover with plastic wrap and allow to rest for about 15 minutes, or up to two hours.  Melt 4 T of the butter, then add the onions and 1/4 t salt and cook until well browned and softened.  Reserve 1/4 c of the onions, chop them finely, and keep them to fill the pierogis with.  (I used the rest of the onions in another recipe, instead of eating them with this recipe.)  Divide the dough into two pieces, roll each out to a very thin dough, and cut the dough into small circles.  You should get about 32 dumplings in all.  Fill the dumplings with your choice of filling, then form them into half moon shapes and crimp them together.  Boil water, adn add salt.  Cook the pierogis until al dente, about 5-6 minutes.  Melt the remaining butter in the skillet used to carmelize the onions, then cook the pierogis 1-2 minutes per side.  Serve.