Heat the oil in a large skillet over medium heat. Add the cumin, coriander, and mustard seeds and cook until they start to sizzle. Add the turmeric powder and mix well.
Add the potatoes and cauliflower and stir to coat with the spices. Cover and cook for 5 minutes.
Add the onion, ginger-garlic paste or freshly grated ginger and garlic, and tomato (if using). Stir to combine.
Cover and cook for 10 minutes or until the vegetables are cooked through.