Cook the dal by boiling or pressure cooking until lentils are soft. (Using a pressure cooker is faster.)
In a frying pan, heat the oil and add mustard seeds. When mustard seeds begin to pop, add onions, ginger, chillies and garlic. Sauté until the onions and garlic are golden brown. Add ground coriander, cumin and chopped tomatoes. Cook and stir until tomatoes are tender.
Add water. Boil 5 minutes. Add cooked dal, stirring well. Add salt to taste, stirring well. Add finely chopped coriander and remove from heat. Serve hot.