In a large saucepan, bring the lentils to a boil in about 1 cup water; reduce the heat to low and let simmer until just tender, about 20 minutes.
Meanwhile, in a large skillet, heat the oil over medium heat and add the onions, carrots and celery and cook until the onions are tender, about 10 minutes. Add the sausage, if using, and cook, breaking up any clumps, until lightly browned, about 8 minutes. Stir in the rice, pistachios, fennel seeds, turmeric, thyme and 1 teaspoon salt; cook until the rice is opaque, about 3 minutes. Stir into the lentils and add 1 3/4 cups water. Bring to a boil, then cover completely and simmer over low heat until cooked, about 45 minutes. Remove from the heat and let stand, without removing the cover, 15 minutes. Fluff with a fork and top with the pomegranate seeds.