Bring 4 cups broth and lentils to boil in large saucepan. Simmer until lentils are just tender, about 30 minutes. Meanwhile, heat olive, oil in heavy large skillet over medium-high heat. Add onion, bell pepper and celery and saute until tender, about 10 minutes. Add onion mixture to lentils. Stir in tomato sauce and salsa. Simmer over medium-low heat until lentils are very tender, about 30 minutes. Thin soup with additional broth if necessary. Stir in spinach and simmer until heated through, about 2 minutes. Season soup to taste with salt and pepper. Serves 6. Bon Appetit October 1991