In a large skillet, cook beef and onion over medium heat until the meat is no longer pink, breaking meat into crumbles; drain. Stir in the water, tomato paste, bouillon and oregano. Cover and simmer for 30 minutes. , Meanwhile, in a large bowl, combine the ricotta cheese, 1 cup mozzarella, Parmesan cheese and egg. Stuff shells with the cheese mixture., Arrange shells in a greased shallow 3-qt. or 13x9-in. baking dish. Spoon meat sauce over shells. Cover and bake at 350° for 30 minutes. Uncover; sprinkle with remaining mozzarella cheese. Bake until cheese is melted, 3-5 minutes longer. Freeze option: After assembling, sprinkle casserole with remaining mozzarella cheese. Cover and freeze unbaked casserole. To use, partially thaw in refrigerator overnight. Remove from refrigerator 30 minutes before baking. Preheat oven to 350°. Bake casserole as directed, increasing time as necessary until heated through and a thermometer inserted in center reads 165°.