For the dal
In a large pot, bring the split peas and 7 cups of water to a boil. Cover, lower the heat, and simmer for 1 hour or until the split peas are tender. I would start checking around the 30-40 minute mark. Drain and set aside.
Meanwhile, combine the garlic, ginger and green chillies in the container of a blender. Add 6 to 8 tbsp wter as needed and blend, pushing down the mixture with a rubber spatula several times, until pureed.
Heat the oil in a wide, medium pot over medium-high heat. Add the onions and stir and fry until they are a rich, reddish brown. Add the tomatoes. Stir and cook until the tomatoes reduce and darken. Add the green spice paste and stir over medium-high heat for 5 minutes. Add the chickpeas and dal, and about 2 cups of water. Stir in the salt, coriander, cumin, garam masala, and tamarind paste, mixing thoroughly. Cover, lower the heat, and simmer gently for 1/2 hour.
Meanwhile make the masala, first boil the potatoes until they are soft and let cool. Peel and chop the potatoes in small pieces.
Heat the oil in a frying pan on medium high. Test the heat by adding one cumin seed to the oil. If it cracks right away, the oil is ready.
Add the cumin seeds and mustard seeds. As the seeds crack, add turmeric, potatoes, green chili, and salt. Stir-fry for a few minutes.
Add lemon juice and coriander.
The potatoes should be slightly moist and not very dry.
Serve both dishes together with flat bread