Heat your oven to 180°C/Fan 170°C/Gas 4. Drain and rinse the chickpeas. Tip into a large bowl. Crush 2 tsp of the coriander seeds in a pestle and mortar or with a spice grinder. Peel and grate or crush the garlic.
Add half the crushed coriander seeds and garlic to the chickpeas. Pour in ½ tbsp olive oil and a pinch of salt. Toss to thoroughly coat.
Scrub the sweet potatoes. Chop them into 2 cm thick wedges. Peel and chop the onions into thick wedges. Add both to the chickpeas and gently mix together. Line a baking tray with baking paper.
Tip the chickpea, sweet potato and onion mix on to the baking tray in a single layer. Roast for 40 mins till the potatoes are cooked through and beginning to caramelise. The chickpeas should be turning crispy.
Roughly chop all the coriander, stalks and all. Finely slice the chilli, flicking out the seeds for a milder hit. Place into a food processor with the reserved coriander seeds and garlic. Roughly chop half of the watercress and add that too.
Pulse everything till you have a thick paste. Slowly pour in 2 tbsp red wine vinegar, ½ tbsp. olive oil and 3 tbsp of cold water while pulsing to make the mojo verde sauce. Season to taste. No processor? Finely chop the coriander, chilli and watercress, whisk in all the other ingredients.
Rub the lamb steaks with ½ tbsp olive oil and plenty of seasoning. Heat a frying or a griddle pan till smoking hot. Fry the steaks for 3-5 mins on each side, depending on their thickness. Cut half way through one - if it’s too pink cook for 1-2 mins more.
Spoon the roasted chickpea, sweet potato and onions onto warm plates with the remaining watercress. Add a lamb steak to each plate, pouring over any juices. Serve with the mojo verde sauce.