Mix together the cumin, salt, garlic powder, pepper, and coat the carne asada with it. Slice into strips, then dice. Add it to a large plastic bag or bowl with a cover, and mix the meat with the Serrano pepper, tomato, onion, and beer. Allow to marinate for at least 15 minutes, but up to 1 hour.
Heat a large skillet with the canola oil over medium high heat, then pour in the entire marinated mixture and allow to cook, stirring occasionally, until all the liquid has evaporated and the meat is cooked through.
Serve in small corn tortillas and top with the onion and cilantro.
*If carne asada is not carried in your local grocery store, you can use top or bottom round, or skirt steak. A thin, lean meat is best.