For the Dough: Mix together the flour and salt, then add the butter and knead it into the dough. Add the water and continue kneading until uniform. Cover and refrigerate for 30 minutes.
For the Filling:
Saute the onions over medium heat. Add beef and salt, and cook until browned.
Add the garlic, butternut squash, tomato paste, and ½ cup water. Cook covered until squash is tender, about 10 minutes. Stir in the greens and red pepper flakes and saute until wilted.
Remove from heat and set aside.
When dough is chilled, divide it into thirds. Roll each third ⅛th of an inch thick. Using un overturned glass (or biscuit cutter), cut rounds in the dough. Repeat with the other pieces of dough. Roll the scraps together and chill them, then roll them out until all the pieces are used.
Place 2-3 Tablespoons of filling in the center of the circle, then pull one side over the other, stretching the dough as you do so. Seal the end with a fork. Repeat until you run out of dough and filling, then brush each empanada with the whisked egg and bake on 350ºF for 30-35 minutes.
Full disclosure: this took me about an hour and a half, from start to finish on my first time! This includes the break to take photos of the ingredients. I was being super careful about measuring everything, and I think if I made the empanadas larger it would be significantly faster. Shoot for 10-12 empanadas and save yourself 20 minutes or so.