Rinse and pat dry 4 pieces sirloin steak. If there are tough tendons around the edges, trim them off.
Make small cuts around the circumference of the beef slices, approximately one notch every 2 inches, with each notch no deeper than ¼ inch. It's simpler than it sounds—just make a vertical cut with a knife or meat cleaver.
Tenderize the slices of beef, then lightly season them with 2 teaspoons salt and ½ teaspoon ground black pepper. Dredge the steaks in 1 Tablespoon all-purpose flour on both sides, and gently shake off any excess flour.
Peel and finely chop 1 large onion.
In a saucepan wide enough to fit all the beef steaks side by side, heat 2 Tablespoon sunflower oil over medium-high heat. Sear the meat on both sides until golden brown, about 1-2 minutes per side. Transfer the seared beef to a plate and set it aside.
Turn the heat down to medium and add the chopped onion. Using a flat wooden spoon, scrape the browned bits from the bottom of the pan while stirring the onions. Add a tablespoon or two of water and continue to work carefully with a flat spoon, dissolving the browned residues in the liquid. Stir until the onions are golden brown.
Return the beef slices to the onion base, including any juices released from the meat. Pour in enough water to submerge the meat halfway. Stir and season with salt and pepper to taste.
Cover the pan with a lid and bring to a boil. Then, reduce the heat to a minimum and let it simmer gently until the meat is tender. The cooking time will vary depending on the type and age of the beef used.
To make the gravy, transfer the beef slices to a plate and keep them warm. Pour the liquid through a sieve, using a spoon to press the remaining onion through into the juice. Salt to taste, if necessary.
Arrange the beef steaks on a plate, pour the finished gravy over them, and serve with a suitable side dish.