1. Cook manicotti shells following package directions; drain and set aside to cool. Heat oven to 350 degrees F. Butter a 13- by 9-inch baking dish.
2. In 2-quart saucepan, melt butter over medium heat. Add onion and sauté 2 minutes. Stir in flour, 1/4 teaspoon salt, and pepper; cook 1 minute. Slowly stir in milk and cook over low heat, stirring constantly, until thickened sauce forms--about 5 minutes. Set aside.
3. In large skillet, heat oil over medium heat. Add garlic and cook 30 seconds. Add spinach and remaining 1/4 teaspoon salt; cover and cook until spinach wilts--about 2 minutes. Remove from heat and stir in beans, mozzarella, dried tomatoes, and 3/4 cup of reserved sauce.
4. With teaspoon, stuff manicotti with vegetable-cheese mixture and arrange in buttered dish. Stir water into remaining sauce and pour over manicotti. Cover with aluminum foil and bake 20 minutes. Uncover, sprinkle with bread crumbs, and bake 15 to 20 minutes longer or until bubbly and bread crumbs are lightly browned. Serve hot.