On medium heat, heat the oil in wok or skillet. Add the red chili (if you want a spicy taste) and chopped onion. Sauté till ¾ brown. Approximately 5-7 mins.
Add the turmeric and cumin powder. Stir a few times, and then add the potatoes. Cook for about 5 minutes, stirring gently to coat the potatoes with masala.
Add the spinach and salt, and then add just enough water to cook the potatoes. Turn the heat down, cover the pan, and cook on low for 5 minutes.
Remove the lid and cook for another 7-8 minutes until the potatoes are cooked and the moisture has almost evaporated. Do not let it dry out.
Once the potatoes are done, remove the skillet from the heat and sprinkle garam masala over the sabzi. Cover and let rest at least 5 minutes for the dish to absorb all the flavors.
Serve with tortillas, rotis, or rice and as an accompaniment to any dal or curry.