To make the alu masala, put the potatoes in a pan with the chopped tomatoes, onion and garlic. Pour over enough cold water to cover the potatoes by 3cm and boil until the potatoes are tender.
Scoop out the potatoes and lightly crush them with a fork – aim to keep the texture rough and chunky. Strain the cooking liquid into a jug, reserving both the liquid and the onion mixture left in the sieve.
Heat the oil in a wok or karahi over a medium heat and fry the cumin seeds and Indian bay leaves or cinnamon for a few seconds, until the cumin releases its aroma.
Add the reserved onion mixture and continue frying for 8-10 mins until the onions have browned. Turn down the heat and add the turmeric, chilli powder and ground coriander and cook for 1 min more, stirring all the time.
Add the potatoes along with the reserved cooking liquid. Simmer for 5 mins, then turn off the heat and stir in the chopped coriander. Cover and leave the flavours to infuse while you make the puris.
To make the puris, sift the flours and salt into a bowl. Work the oil into the flour with your fingers, then add enough cold water to make a very firm dough. Knead for about 5 mins until smooth, then shape into a ball. Put into a lightly oiled bowl, cover, and set aside for at least 30 mins.
Knead the dough again for 1 min, then divide it equally into 20-22 balls, each about 2.5cm in diameter. Slightly flatten the top of each ball and roll out on an oiled surface to 11cm-diameter discs about the thickness of a £1 coin. You can flip the dough over once or twice while you roll.
Fill a karahi or wok with oil until it is no more than two-thirds full and heat to 180C, or until a cube of bread dropped in browns in 30 seconds. Lower a puri gently into the oil. It will sizzle and rise up to the top within a few seconds. Lightly press on the top of it with a fish slice so that it puffs up as it cooks. Fry for 1 min, then flip it over and fry on the other side for 1 min more. Drain on kitchen paper and repeat with the remaining dough balls.
To serve, warm the potato masala through and eat it with the puris alongside.