Cook the Lentils and Vegetables: Pour the water into a large pot and bring to a boil. Add the lentils, onion, carrot, squash, green beans, and tomatoes. Reduce heat and simmer uncovered for 30 minutes, stirring occasionally. Stir in the turmeric and salt. Cook another 10–15 minutes until everything is tender.
Make the Spice Paste: While the lentils cook, heat 1 tablespoon oil in a frying pan over medium heat. Add cumin seeds, coriander seeds, and the dried chili. Fry about 1 minute until fragrant. Add ginger and curry leaves and cook briefly.
Transfer everything to a small food processor or mortar and grind into a coarse paste.
Add the Spice Paste: Stir the paste into the soup and simmer gently for 5 minutes.
Make the Tarka: In the same frying pan, heat 1 tablespoon oil over medium heat. Add mustard seeds and fenugreek seeds. Cook until they start to pop. Stir in the chopped onion and cook about 5 minutes until golden.
Finish and Serve: Pour the tarka into the pot and stir well. Taste and adjust salt if needed. Serve hot with rice, flatbread, or dosas.