In a medium bowl, combine soy sauce, lime juice, 1 tablespoon canola oil, garlic, chili powder, cumin and oregano.
In a gallon size Ziploc bag or large bowl, combine soy sauce mixture and steak; marinate for at least 1 hour up to 4 hours, turning the bag occasionally.
Heat remaining 1 tablespoon canola oil in a large cast iron skillet over medium high heat. Add steak and marinade, and cook, stirring often, until steak has browned and marinade has reduced, about 5-6 minutes, or until desired doneness.
Serve steak in tortillas, topped with onion, cilantro and lime.
Notes
Cut the skirt steak before marinating. Dice the skirt steak into 1/2-inch pieces prior to marinating, not after cooking. This will allow the marinade to coat each piece evenly, yielding a much better sear in the skillet. This is also key to getting that favorited caramelized crust on each piece!
Avoid marinating for too long. The high acid content from the lime juice can break down the muscle fibers if marinated for too long. Marinate the steak for 1 to 4 hours.
Use a cast iron skillet. A cast iron skillet is the same tool used in many street food stalls in Mexico. It retains heat much better than nonstick pans or even a grill grate, yielding a better sear on the meat without steaming it and giving the steak that authentic sear for Mexican street tacos.
Use flour or corn tortillas. Mexican street tacos are traditionally made with corn tortillas but small flour or corn tortillas can be used depending on personal preference.
Double up the tortilla. Street tacos are typically served on two tortillas, known as la copia or "the copy". This practice helps keep the filling together, and if the first tortilla breaks, the back up tortilla can be used for the rest of the filling.
Serve with your favorite toppings. Serve with diced onion, cilantro, cotija cheese, salsa verde, guacamole or salsa. Cotija cheese is a classic Mexican ingredient that adds salty, authentic flavor to any street taco recipe.
Freeze before assembling in tacos. Let the leftover carne asada cool completely; portion into plastic freezer bags in individual servings, squeezing out any excess air before sealing. Label, date and freeze up to 2 months. To reheat, thaw overnight in the fridge, reheating over low heat, stirring occasionally, until heated through.