Pour the couscous into a bowl and cover with 300ml boiling water. Cover the bowl with a large plate or cling film and set aside for 20 mins.
Slice the onion and green pepper into strips. Peel and finely chop the garlic and chilli (flicking out the seeds for less heat). Dice the tomatoes.
Heat a large frying pan until smoking hot and add 1 tbsp olive oil. Season the rump steak well with salt and pepper and add to the pan. Fry for 3-4 mins in total, turning occasionally, until slightly browned on all sides. Transfer the steak to a plate. Set aside.
Slide the onions, pepper, garlic, chilli and 1 tsp each cumin and coriander seeds into the empty steak pan and turn the heat down slightly. Soften for 3-4 mins, stirring occasionally.
Add the tomatoes and 150ml boiling water to the pan. Stir in the steak. Bring to the boil, then cover and simmer for 15 mins.
Squeeze in the juice from half the lemon. Give everything in a pan a stir. The veg should be tender and the steak cooked through.
Slice the rest of the lemon up into wedges. Use a fork to fluff up the couscous. Serve the steak and vegetables on a bed of couscous, with the lemon wedges on the side.