Heat the Oil: In a large pan, heat the oil over medium heat.
Add Spices: Once the oil is hot, add the cumin seeds. Let them sizzle for about 30 seconds.
Sauté Onions: Add the chopped onions and cook until they turn golden brown.
Add Garlic and Ginger: Stir in the minced garlic and grated ginger. Cook for another minute until fragrant.
Cook Tomatoes: Add the chopped tomatoes. Cook until they soften and start to break down, about 5 minutes.
Add Spices: Mix in the turmeric powder, ground cumin, ground coriander, chili powder, and salt. Cook for another 2 minutes, stirring occasionally.
Add Potatoes: Add the diced potatoes and mix well to coat them with the spices.
Add Water: Pour in the water, bring the mixture to a boil, then reduce the heat to low. Cover and simmer for about 15-20 minutes, or until the potatoes are tender.
Add Spinach: Stir in the spinach. If using fresh spinach, cook until wilted. If using frozen spinach, cook until heated through.
Adjust Seasoning: Taste and adjust seasoning if necessary.
Serve: Garnish with fresh cilantro if desired. Serve hot with rice or naan.