Rub the sirloin steak with olive oil and Worcester sauce. Sprinkle with coarse coriander and season generously with salt & pepper. Heat a heavy pan on the hob. Once the pan is hot add a drizzle of olive oil. Place the marinated steaks on the hot pan and sear on both side (cook for ± 5 minutes on each side or to preferred degree of doneness). Season with salt & pepper to taste. Leave to rest for 15 minutes and slice into thin strips.
Heat the olive oil in a saucepan over medium heat. Add the sliced onion petals and season with salt. Sauté the onions until tender. Add the brown sugar and balsamic vinegar. Continue to cook for another 5 minutes until the onions are glazed and the sauce thick.
In a small bowl. Mix the mayonnaise and wholegrain mustard together.
Slice 8 slices of bread. Spread mustard mayonnaise on the inside of the top & bottom slice. Layer the sirloin steak strips, tomato, cheese and rocket on the bottom half. Top with balsamic caramelised onions.