Just before serving, transfer the Raita into a serving bowl. Heat the oil in a small pan, and add the mustard seeds. When they splutter, add the lentils and turn the heat down to low. Stir this until the lentils turn a golden brown. Then add the fenugreek and cumin seeds, and the asafoetida. Stir once or twice and turn off the heat. Add the curry leaves and pour this into the yogurt.