Combine the lentils, water, and lime in a medium saucepan and bring to a rapid boil over high heat. Reduce the heat to low and simmer gently until the lentils are tender but not mushy, about 30 minutes. Periodically push the limes into the surface and squeeze them gently with tongs.
Drain the lentils, discard the limes, and set the lentils aside to cool to room temperature.
Combine the onion, tomatoes, and parsley in a medium bowl, add the cooled lentils, and mix well. Whisk together the oil and lemon juice in a small bowl and stir into the lentil mixture. Season to taste with salt and pepper and serve.
