Heat your oven to 200°C/Fan 180°C/Gas 6. Scrub the sweet potatoes and slice them into chips. Pop them in a bowl with 1 tbsp olive oil and a pinch of salt and pepper. Toss to coat in the oil and seasoning.
Spread the sweet potatoes out on a baking tray. Roast for 25 mins.
While the sweet potato chips cook, finely chop the coriander and parsley, both leaves and stalks. Peel and crush the garlic. Halve the chilli. Scoop out the seeds and white bits. Finely chop it. Put them all in a small bowl.
Add 2 tsp cumin seeds to a dry frying pan and toast for 1-2 mins till they smell aromatic. Add to the bowl. Pour in the vinegar. Season with salt and pepper. Stir to mix. This is your mojo verde.
When the potatoes have been in the oven for 25 mins, put the frying pan back on a high heat. Warm it up for 2-3 mins till it’s smoking hot. Meanwhile, rub ½ tbsp olive oil and some pepper into the steaks. Add the steaks to the smoking hot pan and fry for 1 min on each side (or longer if you prefer your steaks well done). Transfer the steaks to a warm plate or board, loosely cover with foil and rest for 3 mins.
Juice half the lemon into a bowl and whisk in ½ tbsp olive oil and some salt and pepper to make a dressing. Toss the salad leaves with the dressing.
The sweet potato chips should have cooked for 30 mins by now, and be tender and a little charred. Serve the steaks with the sweet potato chips, the salad and the mojo verde on the side for spooning over.