1. Place the stock4 cups, lentils1 cup, celery2, carrot1, thyme1 sprig, and salt1 teaspoon in a medium saucepan and bring to a boil. Reduce heat to low and simmer until the lentils are tender, about 30 minutes, depending on the lentils. (If they begin to dry out, add water as needed.) Remove and discard the thyme. Drain and transfer the mixture to a bowl; let cool.