Heat the oil in a large frying pan or skillet over medium-high heat.
Once the oil is shimmering, add the cumin seeds and let them sizzle for about 5 seconds until fragrant.
Add the chopped onion and sauté for 6–7 minutes, until the edges begin to turn a lovely golden brown.
Stir in the grated ginger and crushed garlic, cooking for another minute until aromatic.
Mix in the tomato purée and ground turmeric, stirring constantly for one more minute to cook out the raw flavor.
Pour in 2 cups / 475 ml of water, scraping up any browned bits from the bottom of the pan.
Add the mung bean sprouts, diced potatoes, sliced mushrooms, salt, and red chili powder.
Stir everything together and bring the mixture to a boil.
Once boiling, reduce the heat to low, cover the pan, and let it simmer gently for 25 minutes, or until the potatoes are fork-tender.
Remove from the heat and stir in the fresh lime or lemon juice just before serving.
Taste and adjust seasoning if needed.