Start by making the Sambar Ketchup, which can be made up to a week in advance and kept covered in the refrigerator. Soften the onion in the oil for a few minutes, then add the garlic, ginger, and chili. Add all the spices and stir together for a minute, then add the rest of the ingredients in order and simmer until thick but pourable: around 30–40 minutes. You may want to add more beer (or water) if it gets too dry. When cooked, season to taste—it should be sharp, sweet, and spicy. Blend until smooth.
Boil the potatoes in salted water until soft – around 15 minutes – then drain.
While the potatoes are boiling, make the Weissbier dosa batter by simply combining everything, apart from the oil, in a bowl and whisking together—it should be the consistency of double cream, so add more beer or water to achieve that if needed.
In a large pan, heat the oil and add the mustard seeds, curry leaves, and cumin seeds, frying until they all pop and sizzle. Then add the onion, chili, and garlic and fry to soften the onion, being careful that it doesn’t burn –this should take around 10 minutes. Add the ground coriander and turmeric, then add the potatoes, using a fork to crush them open and mix them into the spicy onion mix. Cook for 5–10 minutes while you fry the dosa.
Add a little oil to a large frying pan over a medium heat. Take a ladle of the batter and add enough to the pan to form a thin layer, then fry for a minute or two before flipping it over and cooking for another 30–60 seconds. Wrap in aluminium foil to keep warm while you repeat with the rest of the dosa batter.
To serve, take a dosa and place a large spoonful of potatoes in the middle. Add some sambar ketchup, then wrap it up and eat.