Prepare the dough.
Cover and refrigerate while making the filling.
In a medium skillet, heat the oil over medium heat and cook the onion, stirring frequently until very soft and all juices have cooked away, about 10 minutes.
Stir in the salt, chipotles, and epazote.
Cook for 1 minute.
Transfer to a plate and cool completely.
Preheat the oven to 400°.
Place half of the reserved dough on a floured board and roll out thin.
Cut into 4-inch circles.
Gather the scraps and re-roll.
Put 1 tablespoon of the filling on each circle and fold in half.
Moisten the edges with water and crimp with a fork or fingers to seal.
Place on an ungreased baking sheet.
Brush the tops with beaten egg white and bake until golden brown, about 15 minutes.
Cool on a rack.
Repeat rolling, filling, and baking with the remaining half of the dough.