Heat the oven to 220C (200C fan)/425F/gas 7 and line a tray with a reusable baking mat.
Cut each aubergine into an aubergine “octopus”: keeping the top of the aubergine intact, slice down lengthways through the middle, then turn it 90 degrees and slice down lengthways again, to create four “legs”. Slice down each leg in half, to create eight legs.
Arrange the aubergines head to foot on the tray, drizzle over two tablespoons of oil and rub it into all the nooks and crannies. Do the same with a little salt, both inside and outside, then roast for 30 minutes until soft.
While the aubergines are roasting, make the sauce. Heat four tablespoons of oil in a frying pan over a medium-low heat and, once it’s hot, add the shallots, garlic, chillies, lemongrass and ginger, and fry, stirring regularly, for 15-20 minutes, until soft and translucent. Stir in the soy, tamarind, sugar, three-quarters of a teaspoon of fine sea salt and peanut butter, and cook, stirring, for a couple of minutes. Slowly pour in 400ml water, mixing it in as you go, to create a thick sauce.
Once the aubergines have had their roasting time, pour the sauce all over the aubergines, making sure some of it goes inside all the cuts, then pop back in the oven for 10 minutes.