Preheat Traeger Flatrock to medium heat-- using all three zones.
Using a big sharp chef's knife, chop up the leftover steak in to 1/3-1/4 inch cubes.
Then run your knife through the meat again, cutting into slightly finer pieces with irregular shapes.
In a saucepan, combine the canned enchilada sauce, hot sauce, fajita seasoning and beef bouillon (dissolved in a small amount of hot water).
Stir to combine and place on the griddle to begin heating through.
Chop the cilantro and white onion into fine dice.
Shred the Oaxaca cheese.
To the bowl of chopped steak, add 2 large spoonfuls of the papered sauce. Stir to combine.
Transfer the sauced steak to the hot griddle and being searing.
Dip the tortillas in the prepared sauce and put the dipped tortillas directly on the hot griddle, sprayed with cooking oil.
Cook the tortilla for 2 minutes, then flip.
Add a big handful of shredded cheese to the tortilla.
Then top the cheese with a generous portion of the griddled steak.
Sprinkle the steak with chopped onion.
Add more cheese atop and continue cooking on the Flatrock.
When the cheese begins to melt, carefully fold each tortilla in half and press down with your spatula.
Continue cooking the tacos until the cheese is melted throughout, about 5 minutes.
Transfer the cooked tacos to a cutting board or platter.
Sprinkle with chopped cilantro.
Pour some of the heated sauce into a serving bowl.
Serve your quick birria tacos hot off the Flatrock along with a bowl of sauce for dipping.