Forget fast food - Bill's steak sandwich beats any hamburger.
Heat 2 tablespoons of the olive oil in a large frying pan over medium heat, add the onion and cook for 10 minutes, stirring frequently, until golden.
Add vinegar, salt and pepper and cook for a minute longer until caramelised.
Remove onion from the pan and set aside.
Using the same pan, heat the remaining olive oil over high heat.
Add steak to the pan and sprinkle with salt and pepper.
Cook for 1 minute, turn and cook the other side for a further minute.
Spread the bread with garlic crème.
Place rocket on 4 of the slices and top with tomatoes.
Place onions on top of tomato, followed by the steak.
Top with a second slice of bread.
Place egg yolks, vinegar, garlic, salt and pepper in a bowl (or a food processor) and whisk until combined.
Add the oil drop by drop, whisking constantly.
When the sauce starts to thicken, add the oil in a steady stream until the oil is fully incorporated.
If the crème is too thick, thin with a tablespoon of warm water.
Alternatively you can mix ¾ cup of store bought mayonnaise with the balsamic vinegar, garlic, salt and pepper and stir until combined.