Heat oil in a pan over medium heat. Add cumin seeds and let them splutter.
Add finely chopped onions and sauté until the colour slightly changes.
Add slit green chilies and ginger garlic paste and sauté for another minute until the raw smell disappears.
Add finely chopped tomatoes and cook until they turn soft and mushy.
Add turmeric powder, red chilli powder, coriander powder, and salt. Mix well and cook the spice powders for a minute.
Add diced potatoes and green peas. Mix everything until the potatoes and peas are well coated with the onion tomato masala.
Add ½ cup of water. Cover the pan with a lid and cook on low to medium heat until the potatoes are tender and the peas are well cooked, stirring occasionally to prevent sticking.
If the water has fully evaporated and the potatoes remain uncooked, sprinkle more water, close and cook until the potatoes and peas are done. 
Once the potatoes and fresh peas are cooked, sprinkle garam masala powder and mix well.
Cook for another 5 minutes with the pan open, so the potatoes are slightly roasted. 
Garnish with fresh coriander leaves. Serve the hot and quick Indian aloo matar as a side dish with any Indian bread like roti, naan, or basmati rice.