Peel the potatoes and cut them into 1-inch cubes, then rinse and set aside. Dice the onion finely. Mince the garlic and grate the ginger.
Heat the oil in your medium pot over medium heat until shimmering. Add the cumin seeds and let them crackle for about 20 seconds, filling the kitchen with a warm aroma.
Add the diced onion to the pan and cook, stirring occasionally, until it turns golden and fragrant, about 8-10 minutes. You want a rich, caramelized color and a sweet smell.
Stir in the minced garlic and grated ginger, cooking for another minute until fragrant and slightly bubbly, ensuring they don't burn.
Sprinkle in the turmeric, cumin, and coriander. Toast the spices briefly, about 30 seconds, until they release their aroma and turn a warm shade, stirring constantly.
Add the chopped tomatoes and cook, stirring occasionally, for 5-7 minutes until they break down and form a thick, fragrant sauce.
Gently add the potato cubes to the sauce, stirring to coat them well with the spices and tomato mixture. Pour in about 1 cup of water or broth, then cover and simmer on low heat for 15-20 minutes until the potatoes are tender.
Check the potatoes by piercing with a fork; they should be soft and fluffy. If needed, cook a few more minutes, adding a splash of water if the sauce becomes too thick.
Stir in the chopped fresh spinach and cook for 2-3 minutes until wilted and vibrant green. The aroma will be earthy and fresh, balancing the warm spices.
Taste the curry and add salt as needed. For brightness, squeeze in a teaspoon of lemon juice, then let it rest for a couple of minutes to meld flavors.
Serve hot in bowls, garnished with herbs if desired. Pair with rice or crusty bread for a wholesome, comforting meal.
