Put the cauliflower on a baking sheet and toss with 2 tablespoons of the oil, the ground cumin, coriander and 2 teaspoons salt. Roast until soft and lightly browned, about 20 minutes.
Meanwhile, heat the remaining 2 tablespoons oil in a medium Dutch oven over medium heat. Add the onion and cook, stirring occasionally, until lightly browned, about 10 minutes. Add the garlic, chile, ginger and cumin seed, and cook until fragrant, stirring, about 1 minute (add a splash of water if the ingredients begin to stick). Add the potatoes, tomatoes, garam masala, turmeric and 2 teaspoons salt and cook, covered, stirring occasionally to prevent sticking, until the potatoes are tender, about 20 minutes.
Stir in the cauliflower and peas and cook until heated through, about 2 minutes. Stir in the cilantro and lemon juice. Serve.
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