Wash the dahl and soak in water. In a large saucepan, put 3 cups of water on to boil. Wash, drain and chop the spinach finely. When the water boils add the drained dahl, salt turmeric and chili powder. Return to the boiling water and cook 5 minutes. Stir in the spinach and begin cooking over medium heat. Meanwhile, heat the ghee in a skillet. Fry the onion, cumin and mustard seed until the onions is golden. Stir this into the cooking dahl and spinach. Add the garam marsala. Cook over low heat until the dahl is is soft and the moisture is almost gone. If it gets too dry soon, add a small quantity of water form time to time.