Combine the potatoes, mustard, cumin seeds, turmeric, 1 tsp each salt and black pepper, the chilli flakes and oil in a large bowl. Tip into a flameproof dish (cast iron works well) along with 100ml water, and cover tightly with foil.

Put the dish on the grill directly over the coals and cook for 30 mins, stirring after 15 mins. Remove the foil and use a fish slice to turn over the potatoes (they should be tender), scraping up any stuck spices. Move the dish to the indirect heat and cook for 15 mins to brown.