Sort through the lentils to remove any small stones or pieces of dirt, and then rinse with cold water in a colander. Add the lentils to a medium saucepan and fill about 3/4 full with water. Bring to a boil over high heat, then cover the saucepan, turn the heat down to a simmer, and cook until the lentils are tender but not mushy, about 25 to 35 minutes, stirring occasionally and adding more water as necessary. Strain the lentils, then rinse under cold running water, and cool completely.Whisk together all ingredients for the dressing in a large bowl, then toss in the lentils, kale, onion, and apple.Serve or store in the fridge for the next day.