Heat the oil in a pot, turn the heat down to medium and add the mustard seeds. When they splutter, add the lentils and sautxe9 till they turn light golden. Add the asafoetida powder, stir once, then add the curry leaves and the chopped onions. Sautxe9 them until they turn translucent, then add the other vegetables except the tomatoes. Stir fry them for about 2 to 3 minutes, then add the tamarind water and the turmeric powder to the pot. Add a little salt as well.