Place the beef in a zippy bag and add onion, oregano, pepper, vinegar, sugar, ½ cup of water and salt.
Marinate for an hour at room temperature. Separate the meat and onions from the marinade liquid. Reserve the liquid and the onions separately.
Lightly pat dry the beef slices using a paper towel.
Heat the oil over high heat in a large non-stick pan. Add the beef slices (careful with splatters), Cook for half a minute, turn and cook the other side for another 30 seconds. Remove from the pan.
Add the onions to the pan and lower the heat to low. Cook and stir until the onions become translucent.
Once the onions are cooked, increase the heat to medium and add the liquid from the marinade and simmer until the liquid has reduced to half. Try and season with salt to taste if you find it necessary. Remove from the heat.
Return the beef to the pan, mix and serve immediately.
If your family or guests are enthusiastic carnivores, you might need to double the ingredients, this one serves about 2 thin slices per guests. You will also need a very large pan, or cook the meat in two stages. Don't let it linger in the pan or it will get very dry and hard.