Peel the potatoes and dice them into 1cm cubes. Place in a pot with water, bring to a boil on the stove, and cook for 10 minutes until soft; drain and set aside (this is part of cooking time).
While potatoes boil, prepare the batter: In a mixing bowl, combine 300g Dosa mix flour with 600ml water. Whisk until smooth and lump-free; let it rest for 5 minutes to thicken slightly.
Finely slice the onions into thin strips. Dice the tomatoes into small pieces. Finely chop the green chilies. Grate the fresh ginger.
Heat 1 tbsp rapeseed oil in a pan over medium heat. Add mustard seeds and cumin seeds; let them splutter for 30 seconds.
Add sliced onions, chopped green chilies, grated ginger, and curry leaves to the pan. Sauté for 3-4 minutes until onions are translucent.
Stir in diced tomatoes and turmeric; cook for 2 minutes until tomatoes soften. Add the boiled potato cubes and salt; mix well and cook for another 3 minutes to blend flavors. Mash lightly with the spatula for a chunky texture. Remove from heat.
Heat the pan (or crepe pan) over medium-high heat with a few drops of remaining rapeseed oil. Pour a ladleful of batter (about 100ml) and spread thinly into a circle using the back of the ladle. Cook for 1-2 minutes until edges crisp, then flip and cook for 1 minute more. Repeat for 6 dosas total (2 per portion).
Place a portion of potato filling in the center of each dosa, fold, and serve hot.