Place a large cast iron skillet over high heat and add the vegetable oil. Heat until the pan is very hot.
Add the beef (and liquid smoke if using) into a pan. Use tongs to turn the steak occasionally. When the beef is cooked about halfway, add the salt. Continue turning until it is browned on the outside.
Once the steak is browned, add the black pepper and beef bouillon (cube). Stir until the steak is coated completely. Add the mustard and mix again.
Use a slotted spoon to remove the beef to a separate plate leaving the juices in the pan. Set the plate aside.
Add the raw onions into the pan and saute. Cook for about 8-10 minutes, until the onions begin to soften but haven’t softened all the way. The onions should still have a little bit of crunch to them.
When the onions are your desired consistency, add the beef back into the pan and saute just long enough to let it warm up again. Turn the heat off, and add the desired amount of cayenne pepper.
Serve with french bread and more mustard on the side. Enjoy!