Preheat oven to 425°F (220°C) and line a baking sheet with aluminum foil.
Put potatoes and cauliflower on a baking sheet and mix with 3 tablespoons of oil. Spread out evenly and roast for 12 minutes, stir and then roast for another 10 to 12 minutes, or until browned but still slightly crisp. Set aside to cool.
While the vegetables are roasting, add ghee or oil to a large sauté pan or wok over medium-high heat. When the pan is hot, add the cumin seeds and cook just until they turn a darker shade of brown, about 30 seconds to a minute.
Reduce heat to medium and stir in the turmeric, and then add the onions and cook until soft and translucent; about 5 to 7 minutes. Add a bit of water if the onions are sticking.
Add the ginger garlic paste, salt, chili powder, and asafetida (if using), and cook for another minute. Stir in the cauliflower and potatoes, mixing gently to coat with the onions and spices. Turn the heat to low; cook uncovered for 5 to 6 minutes more, or until the vegetables are tender but not mushy, stirring occasionally. If the mixture is sticking to be bottom of the pan, add a tablespoon or two of water.
Remove from the heat and add lemon juice. Stir to combine, and taste for salt, chili, and acid. Garnish with cilantro and serve.
Traditionally for cauliflower potato curry, the potato and cauliflower are either blanched before adding to the curry, or they are cooked in the tomato sauce. This process takes longer than I have the patience for and requires attention at the stove. Roasting the vegetables in the oven only requires one stir in the middle of the cooking time, otherwise, it is is hands-off, and roasting lends the vegetables flavorful, toasted notes—a win-win.