Heat the oven to 500°F with a rack in the middle position. In a large bowl, stir together the melted ghee, 1 teaspoon of the garam masala, ¼ teaspoon of the cumin and ½ teaspoon each salt and black pepper. Add the cauliflower and toss to coat. Spread in an even layer on a rimmed baking sheet, cut sides down. Roast, without stirring, until deeply browned and tender, 18 to 20 minutes.

Meanwhile, in a blender, combine the tomatoes with juices, cashews and ¾ cup water. Puree until smooth, 1 to 2 minutes; set aside. In a Dutch oven over medium, melt the remaining 2 tablespoons ghee. Add the onion and ¾ teaspoon salt; cook, stirring, until softened and beginning to brown, 3 to 5 minutes. Add the garlic and ginger; cook, stirring, until fragrant, about 30 seconds. Add the remaining 1 teaspoon garam masala, remaining 1 teaspoon cumin and pepper flakes; cook, stirring, until the mixture sticks to the bottom of the pot, about 1 minute.

Add the tomato-cashew puree. Bring to a simmer, scraping up any browned bits, then reduce to medium-low and cook, uncovered and stirring, until slightly reduced and thick enough to coat the back of a spoon, 8 to 10 minutes. If the cauliflower is not done, remove the pot from the heat and set aside, partially covered. Stir in the lemon juice. Stir in the cauliflower and peas. Cook over medium, stirring, until hot, about 2 minutes. Taste and season with salt and black pepper.