To reduce the salt content of the dish, try swapping the salt for paprika or lemon juice.
Heat the oil in a pan. Add the cumin seeds, chopped onion and garlic, and fry until softened and starting to brown.
Add the chopped tomatoes and green chillies, and cook for another minute. Turn the heat down, add the cumin powder (if using) and turmeric, mix well and fry for a further minute.
Add the potato, and then the spinach a minute or 2 later. Stir well to coat the potato and spinach in the spices, then cover the pan and leave to cook on low for 10 to 15 minutes, until the potato is soft. Stir occasionally to avoid sticking.
Remove the lid and turn the heat up. Once the rest of the water has evaporated, turn off the heat and leave to cool for a few minutes.