Remove the stalks from the chillies, then blitz in a spice grinder or mini blender until finely broken down – some flakes are fine, but there shouldn’t be any large chunks. Transfer to a bowl.
Toast the cumin seeds in a small frying pan over a medium heat for 1-2 mins or until aromatic. Set aside ½ tsp and grind the rest to a powder using a pestle and mortar. Add to the bowl with the chillies, along with the sugar, cocoa, cinnamon, salt and ¼ tsp of black pepper.
If the thickness of the steak is uneven, gently pound to roughly even out – it doesn’t have to be perfect. Rub the steak with the oil and season lightly with salt. Rub 3-4 tbsp of the spice blend over the steak and leave to marinate at room temperature for 30 mins. Meanwhile, combine the onion and lime juice with 1 tsp of salt and the reserved ½ tsp of cumin seeds. Let it stand, tossing occasionally, as the steak cooks.
Heat the oil in a large cast-iron pan over a medium-high heat and, once hot, cook the steak for 5-6 mins on each side or until charred on the outside and medium-rare inside – this will depend on the thickness of the steak. Transfer to a platter, loosely cover and leave to rest for 5 mins.
Drain the onions and toss with the coriander. Finely slice the steak across the grain and serve with the onions and a green salad.
