Vegetable Lasagna with Gruyere and Walnuts

Preheat oven to 350 F. In 12-inch skillet cook onion, carrot, celery, sweet pepper, and garlic in olive oil 5 minutes or until tender. Stir in spinach. Cook and stir 1 minute. Stir in tomato puree, Italian seasoning and salt. Meanwhile, cook lasagna noodles according to package directions. Drain noodles; rinse with cold water. Drain well; set aside. For filing, in bowl combine ricotta cheese, eggs, and walnuts; set aside. Spread about 1/2 cup tomato sauce over bottom of 3-quart rectangle baking dish. Layer one-third of cooked noodles in dish, overlapping as needed. Spread with half the filling, 1 1/3 cups vegetable mixture, and 1 1/3 cups Gruyere cheese. Repeat layers. Top with remaining noodles. Spoon any remaining vegetable mixture over the noodles. Sprinkle any remaining cheese. Bake, covered, 45 minutes. Uncover ; bake 10 minutes more or until heated through. Let stand 10 minutes before serving.