Heat the vegetable oil in a large skillet over medium heat, then add the cumin seeds and let them sizzle until fragrant, about 30 seconds.
Add the chopped onion to the pan and cook, stirring occasionally, until it becomes translucent and starts to turn golden, about 5 minutes.
Stir in the minced garlic and cook for another minute until fragrant, filling the kitchen with a warm aroma.
Sprinkle in the turmeric powder, salt, black pepper, and cayenne pepper (if using), stirring to evenly coat the onion mixture with spices.
Add the diced tomatoes along with their juice, stirring well and allowing the sauce to simmer until it starts to thicken and darken, about 10 minutes.
Place the peeled and cut potatoes into the skillet, stirring gently to coat them with the tomato gravy.
Pour in the water or broth, bringing the mixture to a gentle simmer.
Reduce heat to low, cover the pan, and let everything simmer slowly for about 25-30 minutes, or until the potatoes are tender and the gravy is glossy and thickened.
Uncover and gently stir the curry; check if the potatoes are soft and easily pierced with a fork.
Adjust seasoning as needed and cook for another 2-3 minutes uncovered to meld flavors.
Serve the Dopiazeh Aloo hot, with a side of bread or rice for a complete meal.
