Heat the vegetable oil in a large pan or skillet over medium heat. Once hot, add the cumin seeds and cook for 30 seconds until they begin to sizzle.
- Add the chopped onion and sauté for 5 minutes until softened and golden brown.
- Stir in the garlic, ginger, and green chili (if using) and cook for another 2 minutes until fragrant.
- Add the cubed potatoes, peas, turmeric, ground coriander, garam masala, ground cumin, and red chili powder (if using). Stir well to coat the vegetables in the spices.
- Pour in the water or vegetable broth and bring to a simmer. Cover and cook for about 15 minutes, or until the potatoes are tender.
- Once the potatoes are cooked, add the spinach in batches, stirring until it wilts. Continue to cook for another 5-7 minutes until the spinach is fully incorporated and tender.
- Season the curry with salt to taste and stir in the lemon juice for a burst of freshness.
- Serve hot with rice, roti, or naan. Enjoy!