Step 1 – Heat oil in a frying pan and brown the onion. Season, remove from pan and set aside.
Step 2 – In the same pan, heat the remaining oil and cook the steak to medium rare. Remove from the pan and set aside.
Step 3 – Fry an egg in a nonstick pan. Season.
Step 4 – Assemble the sandwich: Slice bread in half and toast it. Layer steak on bottom bun, followed by the onion, then the fried egg. Sprinkle with salt and top with other half of bun.