Heat vegetable oil in a large saucepan. Add the chopped onion and one teaspoon of cumin seeds to the oil. Stir together and cook until onions become creamy, golden, and translucent. Add chopped coriander stalks, two teaspoons of turmeric, and one teaspoon of salt. Add chopped chillis (according to taste) Stir tomatoes into onion mixture. Add ginger and garlic; mix thoroughly. Add potatoes and cauliflower to the sauce plus a few tablespoons of water (ensuring that the mixture doesn't stick to the saucepan). Ensure that the potatoes and cauliflower are coated with the curry sauce. Cover and allow to simmer for twenty minutes (or until potatoes are cooked). Add two teaspoons of Garam Masala and stir. Sprinkle chopped coriander leaves on top of the curry. Turn off the heat, cover, and leave for as long as possible before serving.