In a large bowl, season steak with salt, pepper, garlic, cumin, oregano, and Sazon. Massage the steak with the seasonings. Add in onions and naranja agria. Let it marinate for 15 minutes.
Heat oil in a large pan over medium high heat. Add in steak and onions. Cook steak for 4-6 minutes per side, or until desired level of done. May be done in batches. Leave the onions until as softened as desired. To assemble the sandwiches, cut steak one steak into 3-4 slices and place it on a cut 4 inch Cuban bread that has been smeared with mayonnaise. Top with onions and close the sandwich. Serve.