Cut the tomatoes into cubes. Mince together the ginger and the red chili.
Melt half butter in a large pan, add turmeric powder and yellow mung dal. Fry 1 minutes, then add diced tomatoes.
Mix and add water, salt, minced ginger and red chili. Cook over low heat 1 hour, until creamy.
In a skillet melt the remaining butter, add cumin and black mustard seeds and cook until the mustard seeds start to pop. Add to the dal and serve hot.