Heat the oil in a large deep skillet or Dutch oven over medium heat. Add the cumin seeds and cook for about 30 seconds, until fragrant. Add the onion and cook, stirring often, until softened and lightly golden, 6 to 8 minutes.
Stir in the garlic and ginger. Cook for 30 seconds, then add the chopped tomato, ground coriander, ground cumin, turmeric, Kashmiri chili powder or paprika, optional cayenne, and salt. Cook for 2 to 3 minutes, stirring, until the tomato softens and the spices are fragrant.
Add the potatoes and cauliflower. Stir gently until the vegetables are well coated in the spiced tomato mixture. Pour in the water and bring it to a gentle simmer.
Cover the skillet, reduce the heat to medium-low, and cook for 18 to 22 minutes. Stir once or twice during cooking so the vegetables do not stick. Add a splash of water if the pan becomes dry before the potatoes are tender.
When the potatoes are tender and the cauliflower is cooked through but still holds its shape, stir in the optional peas. Cover and cook for 2 to 3 minutes, until the peas are hot.
Remove from the heat. Stir in the cilantro and lemon juice, then taste and add more salt if needed. Serve warm with basmati rice, naan, roti, or plain yogurt on the side.