1. In a heavy 3- to 4-quart casserole, heat the vegetable oil over moderate heat until a light haze forms above it. Add the dal and stir for a minute or so, then drop in the mustard seeds and, when the oil begins to sputter, add the onions.
2. Add the hot chili and, stirring constantly, pour in the farina in a slow, thin stream. Still stirring, fry for about 5 minutes, until the farina is lightly browned. Stirring well after each addition, add the tomatoes, carrots, scallions, water and salt. Bring to a boil over high heat, reduce the heat to the lowest possible point, cover tightly, and cook for 10 minutes, or until the farina is thick and the vegetables tender.
3. Taste for seasoning. To serve, pour the uppama into a heated bowl and sprinkle it with the coriander and lemon juice.
