Combine Ginger-Garlic Paste/coriander/turmeric/ ½ c water in a small bowl and mix well.
Warm the oil in a large pot over med-high. Add serrano and cook for 30 seconds. Add cumin seeds and cook another 30 seconds. Add the Ginger-Garlic mixture/½ c water. Cook until slightly thickened and darker in color (~2 minutes).
Add potatoes. Stir to coat. Add salt and ½ c water. Cover. Cook over med for 20 min, stirring occasionally and adding more water if necessary.
Add cauliflower and ½ c water. Recover and continue simmering (20-40 min) until vegetables are tender and sauce is thick, stirring occasionally and adding more water if necessary.