Heat the oil in a heavy-bottomed pan over medium heat and add the cumin seeds. Let them crackle and become fragrant, about 30 seconds.
Add the minced garlic and grated ginger to the pan. Sauté, stirring often, until fragrant and just starting to turn golden, about 1 minute.
Stir in the ground coriander, turmeric, and chili powder. Toast the spices for 30 seconds until their aroma fills the air and they turn a slightly darker shade.
Add the chopped tomatoes to the pan. Cook, stirring occasionally, until the mixture thickens slightly and the oil begins to shimmer around the edges, about 5-7 minutes.
Gently add the diced potatoes to the tomato-spice mixture. Stir well to coat the potatoes evenly.
Pour in about 1 cup of water or vegetable broth, then cover the pan and reduce the heat to low. Simmer for 10 minutes, until the potatoes are starting to soften.
Uncover the pan and add the broccoli florets. Stir gently to combine, then continue cooking uncovered for another 5-7 minutes, until the broccoli is bright green and crisp-tender, and the potatoes are fully cooked.
Taste the curry and season with salt and a squeeze of lemon juice. Adjust spices if needed to balance the flavors.
Remove from heat and sprinkle with chopped cilantro. Serve hot, ideally paired with rice or flatbread.
