Mix onion, chipotle pepper, garlic, cumin, salt, and rub in a bowl on the beef.
Set aside for 10 minutes.
Preheat the gril or an iron pan to high heat. Add the steak, lower the heat to medium high. Cook for 1 to 2 minutes or until, lightly browned. Turn, cover and cook for 3 to 5 minutes more. (medium cooked)
Remove from heat, cover with foil 5-10 minutes and serve.