Bring 4 cups of water to a boil in a medium saucepan. Add onion, cactus, cilantro sprigs and salt and  boil over medium-low heat for 15 minutes or until cactus pieces are tender. Rinse and drain.  Discard onion and cilantro. Set aside.

Preheat oven to 400° F.
Cook soy (or pork) chorizo in a medium frying pan over medium heat.  Break up with large spoon into small crumbles. Stir in cactus pieces  Set aside.

Unravel Oaxaca cheeses and place in a medium cast iron skillet (or two small cast iron skillets).  Place in oven and bake for 15 minutes.  Remove from oven, top with chorizo/cactus pieces and return to oven.  Bake for 5 minutes more.  Remove from heat.

Enjoy with warm corn tortillas or Tortilla chips.