For the masala aioli
Preheat the oven to 200°C, gas mark 6.
Rest each potato on a large serving spoon and cut widthways at 3mm intervals – cutting the potato on a spoon stops you slicing all the way through. Repeat with all the potatoes.
Roughly grind the peppercorns with a pestle and mortar.
Melt the ghee in a large roasting tin. Once sizzling, add the potatoes and pared lemon zest.
Toss well so that all the potatoes get coated in the fat, then season well with the salt and the freshly ground black pepper.
Roast on the bottom shelf of the oven for 1 hour-1 hour 15 minutes until golden and tender.
Discard the lemon zest.
To make the aioli, mix all the ingredients together.
Serve the potatoes with the aioli.