Heat the oil and butter in a large skillet or pan over medium heat. Add the cumin seeds and let them sizzle for about 1 minute.
Add the chopped onion to the skillet and sauté until it becomes translucent, about 5 minutes. Then, add the minced garlic and grated ginger, and cook for an additional minute until fragrant.
Stir in the ground turmeric, ground coriander, and chili powder, and cook for another minute to toast the spices.
Add the diced potatoes to the skillet and toss them well to coat with the spice mixture. Cook for about 5 minutes, stirring occasionally.
Once the potatoes have started to soften slightly, add the water, garam masala, chopped tomatoes, and frozen peas to the skillet. Mix well to combine.
Reduce the heat to low, cover the skillet, and let the curry simmer gently for 45 minutes to 1 hour, or until the potatoes are tender and the peas are cooked through.
Garnish with freshly chopped parsley or lime juice before serving.
Serve the Aloo Matar hot with steamed rice or warm naan bread for a complete and satisfying meal.