In a large saucepan, heat up the oil over medium heat. Once the oil is hot, add the onion and garlic and stir-fry until soft and translucent, 1-2 minutes. Add the ginger, chili, and cumin seeds, and stir-fry for another 1 minute.
Add the potatoes and cauliflower and cook, stirring frequently, for about 4-5 minutes. Add the sea salt, ground turmeric, and 1 ½ cups of water. Stir to mix all the ingredients well. Bring to a boil and reduce the heat to a simmer. Cover and cook, stirring occasionally, until the potatoes and cauliflower are fork tender and the liquid has been reduced, about 15-20 minutes. You may need to add more water if it gets too dry, but this dish is meant to be thick.
Add the tomato and cook for another 2-3 minutes. Sprinkle in the garam masala and chopped cilantro and toss to mix.
Serve immediately with the cooked basmati rice.
It's important to make sure the cauliflower florets are cut slightly bigger than the potato cubes in order for the two veggies to cook evenly.