Heat your oven to 180°C/Fan 160°C/Gas 4. Scrub and slice the sweet potatoes as thinly as possible. Lay the slices in a medium sized baking dish.
Heat a frying pan over a medium heat and tip in the peanuts. Toast for 3-4 mins till golden. Take care not to let them burn. Tip onto a chopping board and roughly chop.
Finely chop the chilli, flicking out the seeds for a milder heat. Peel the ginger by scraping it with a teaspoon and roughly chop. Peel and roughly chop the garlic. Zest and juice the lime. Roughly chop the coriander stalks, saving the leaves for later.
Tip the peanuts, chilli, ginger, garlic, coriander stalks, lime zest and half the juice into a food processor. Squeeze in the curry paste and pour in the coconut milk. Blitz to combine. Taste and season.
Pour the fragrant coconutty sauce over the sweet potato. Tumble together with your hands, then spread out. Dot the tomatoes over the top.
Cover the dish with foil and seal the edges tightly. Cook for 30 mins. Remove the foil and cook for a further 20 mins till golden. Check the sweet potatoes are cooked by piercing with a knife. If still firm, return to the oven and cook for another 5-10 mins.
Whisk the remaining lime juice with 1 tbsp oil and some seasoning. Pour over the lamb’s lettuce and toss together. Serve with the sweet potato gratin, garnished with the coriander leaves.