Heat oil in a large pan or kadhai over medium heat. Add cumin seeds and let them splutter.
Add chopped onions and sauté until they turn golden brown.
Add ginger-garlic paste and green chili (if using). Sauté for a minute until the raw smell disappears.
Add chopped tomatoes and cook until they turn mushy and oil starts to separate from the masala.
Now, add turmeric powder, red chili powder, coriander powder, and salt. Mix well and cook for another minute.
Add cubed potatoes and cauliflower florets to the pan. Mix gently to coat the vegetables with the masala.
Cover the pan and cook on low heat for about 15-20 minutes or until the potatoes and cauliflower are tender, stirring occasionally.
Once the vegetables are cooked through, sprinkle garam masala and mix well.
Garnish with freshly chopped coriander leaves.
Serve hot with roti, naan, or rice.