For the salsa: Boil the tomatillos in water until soft. Transfer to a blender, along with the serrano peppers, garlic, and salt and pepper to taste. Blend well.
For the tacos: Boil the potatoes in salted water until tender; don't overcook--they should stay in one piece. Smash the potatoes with the butter, being careful to leave good texture; don't mash them all the way as if you were making mashed potatoes. Add the epazote and salt and pepper to taste; mix.
Heat the tortillas on a griddle until soft. Add about a spoonful of the potato mixture to one side of each tortilla, then roll up the tortillas like flautas.
Deep-fry the rolled tortillas in the oil until golden brown.
Serve 3 tacos per person, cover with salsa, and sprinkle with cotija and onions. Serve with a spoon on the side, so people can eat the salsa!
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.