Rinse the lentils in a bowl of cold water. Drain well. Bring 3 cups of water to the boil in a covered pan over a high heat. Add lentils, stir well. Reduce heat to medium and boil lentils, half covered with pan lid, for 10 minutes or until tender. Drain well.
Heat the oil in a heavy-based pan over a medium heat for 1 minute. Add the mustard seeds and cook them just until they start to "pop". Add the cumin, coriander, cardamom and tumeric, stir over the heat for 1 minute.
Add the onion and garlic and cook for 5 minutes, or until soft, stirring occasionally. Add the stock, lemon juice, mint and lentils. Bring to the boil stirring continuously.
Place the tentil mixture in a serving bowl. Serve as a dip, either warm or cold, with crisp pappadams and vegetables.