Heat a splash of oil in a frying pan on a medium heat and fry the onion for about 5 minutes, until soft and sweet. Add the mustard seeds and stand back while they pop. Scoop out a heaped tablespoon of the onion mixture and put to one side to cool.
With the pan still on the heat, add the turmeric and curry leaves and fry for another minute or so, then put the whole lot into a bowl to cool slightly. Add the mashed potato to the onions, then season and mix well. Divide the mixture into 4 portions and shape them into 4 potato cakes. Put them into the fridge to chill while you do a couple of other little jobs.
In another bowl, mash the avocados with the lemon juice (you can use a potato masher here), then stir in the tablespoon of the onion you set aside. Mix, then season well.
To make your pickle, put the sliced cucumber into a bowl and add all the other pickle ingredients. Using your hands, scrunch the cucumber slices to get the favours going.
Now put your frying pan back on the heat. Take the potato cakes out of the fridge and fry them gently and carefully in a little oil for about 2–3 minutes on each side, until warmed through and crispy brown.
Serve each dosa cake piled with the mustard seed, onion and mashed avocado and with a sprightly spoonful of pickle on the side.