Process chillies, garlic, coriander stalks, half the lemon juice, and 50ml olive oil.
Blend until a smooth harissa paste forms.
Flatten beef between cling film with a mallet to half its thickness.
Brush the beef with harissa and olive oil.
Marinate the beef for ten minutes.
Cook couscous according to packet instructions until fluffy.
Mix remaining olive oil, lemon juice, zest, and coriander leaves into the couscous.
Toast cumin and coriander seeds in a hot pan for one minute.
Grind the toasted seeds roughly using a pestle and mortar.
Stir the ground spices and olives into the couscous.
Sear beef in a hot griddle pan for one minute per side.
Rest the beef for one minute then slice it thickly.
Serve beef over the couscous with a drizzle of harissa.