In a skillet, heat the cooking oil over medium heat. Add the diced onion and sauté until translucent, about 2-3 minutes.
Once the onions are softened, add the diced or shredded leftover beef brisket to the skillet. Stir to combine with the onions.
Open the can of chipotle chiles in adobo sauce. Carefully chop a couple of chiles and add them along with a tablespoon of the adobo sauce to the skillet. Stir well to coat the brisket and onions.
Pour in the beef broth and low sodium soy sauce. Stir to combine. Simmer the mixture for about 5 minutes, allowing the flavors to meld.
In a small bowl, mix the cornstarch with a tablespoon of water to create a slurry. Add this to the brisket mixture and cook for an additional 2-3 minutes until the sauce thickens slightly.
While the brisket is finishing, warm the corn tortillas in a separate skillet or directly over a low flame until they are pliable.
To assemble, place a generous scoop of the brisket mixture on each tortilla. Top with chopped cilantro, pico de gallo, a drizzle of crema, avocado slices, coleslaw, shredded lettuce, and jalapeños, according to your preference.
Serve the tacos immediately while they are warm, and enjoy the explosion of flavors in every bite!