Heat oil in a large pan over medium heat. Add cumin seeds and let them splutter. Add chopped onion and sauté until translucent and soft.
Stir in the chopped tomatoes. Sauté for 1-2 minutes until they begin to soften.
Sprinkle turmeric powder and red chili powder over the mixture. Stir well to coat the onions and tomatoes with spices.
Add the chopped spinach and mix it well into the masala.
Pour in 1/2 cup water. Cook uncovered on medium heat, stirring occasionally. Let the spinach wilt and the mixture thicken, about 5–10 minutes.
Garnish with fresh cilantro leaves. Serve hot with naan or steamed rice.