To make the pickled onions, put the sliced onion in a small bowl and mix with the sugar and vinegar. Set aside and allow to marinate while you make the broth.
To make the broth, heat the ghee in a large pan. Fry the shallot and garlic for 3–4 minutes, or until softened. Add the ginger and Jerk seasoning and cook for a few seconds more, stirring constantly.
Pour in the beef broth and add the soy sauce. Bring the liquid to a gentle simmer and cook for 3–4 minutes. Adjust the seasoning to taste, adding a little extra soy sauce if needed.
Preheat a cast iron griddle over a high heat.
For the steak, season the beef on both sides with a little salt, black pepper and Jamaican Jerk rub. Griddle the steak on each side for 1½–2 minutes, depending on its thickness. Transfer to a board and leave to rest for 4 minutes.
Cook the noodles in a pan of boiling water for 3–4 minutes, or according to the packet instructions, stirring occasionally. Drain and place into a warmed, wide shallow bowl.
Ladle the broth on top of the noodles. Slice the steak and arrange it in the bowl. Drain the pickled onions and scatter on top then finish with fresh coriander and chilli flakes.