1. Combine the coconut and lemon juice in the jar of an electric blender and blend at high speed for 1 minute. Stop the machine and scrape down the sides of the jar with a rubber spatula. Blend again until the coconut is reduced to a smooth puree. Then add the onions, ginger root, chili and salt, and blend for about 30 seconds longer.
2. In a heavy 8-inch skillet, heat the vegetable oil over high heat until it begins to smoke. Drop in the mustard seeds, fry for 10 seconds, and add the dat. Stirring constantly, fry for 1 minute to coat the beans evenly with the oil, then pour the entire contents of the blender jar into the skillet and cook for 1 minute longer.
3. Do not let the coconut brown. Serve at once, or cool and serve either at room temperature or chilled. Tightly covered and refrigerated, nariyal chatni can be kept for 2 or 3 days.
