Place the chillies, garlic, coriander stalks and the juice of half a lemon into a food processor with 50ml/1¾fl oz olive oil.
Process until a paste is formed - this is the harissa, the spicy paste that will flavour our beef.
Place the beef between two pieces of cling film and flatten with a meat mallet until it is half its starting thickness.
Brush with some harissa and extra olive oil.
Marinate for ten minutes.
Cook the couscous according to the packet instructions - it should be fluffy and light, not sticky.
Add the rest of the olive oil, the remaining juice and zest of the lemons, and the chopped coriander leaves to the couscous.
Heat a small frying pan until hot, then add the cumin and coriander seeds and toast for one minute.
Place the cumin and coriander seeds into a pestle and mortar and grind roughly.
Add the spices to the couscous along with the olives.
Heat a griddle pan until hot then sear the beef for one minute on each side.
Remove from the pan and rest for a minute before slicing thickly.
Place the couscous on a plate, top with the sliced beef and serve with a drizzle of harissa.