Add the salt, garlic powder, dried oregano, cumin and black pepper to a small bowl. Stir to combine.
Place the bistec de palomilla on a sheet pan or large platter, season them on both sides with the prepared seasoning mix. Squeeze the lime over the steak, on both sides. Refrigerate the steaks for about ½ an hour.
Heat the oil in a large skillet over medium-high to high heat.
When the oil is hot add 2 of the steaks to the skillet so they are not overlapping. Cook the steaks about 1-2 minutes on each side, depending on the thickness. Make sure the steak is browned on the outside, and then continue cooking to the desired doneness.
Remove the skillet from the heat, place the steaks on a plate or pan. Keep the steaks warm by covering loosely with aluminum foil, or place them in the microwave or oven to keep away from drafts. Cook the remaining steaks.
Once all of the steaks are cooked, do not wash the skillet. Immediately add the sliced onions, place the skillet back over medium-low heat. There should be enough residual heat in the skillet to cook the onions. Stir frequently and cook until they start to soften.
Serve the Bistec de Palomilla with white rice and black beans, if desired.