Heat oil in a medium-sized pan over medium-high heat. Splutter cumin seeds. Add garlic, and sauté for few seconds. Add onions, and sauté, stirring occasionally, for 2-3 minutes or until it becomes soft and translucent. Add tomatoes and green chillies (if using), and sauté, stirring occasionally, until it starts to break down.

Reduce heat to low. Add ground turmeric, ground coriander, chilly powder, and garam masala. Saute for few seconds. Add potatoes and cauliflower, and mix well. Increase the heat to medium. Cook, covered, stirring occasionally, for 5-6 minutes. When done, sprinkle garam masala, and mix well. Remove from heat. Garnish with coriander leaves. Serve hot with rice or roti.