In a large skillet over medium high heat brown the beef and sausage. Remove grease. Add meat to a food processor and pulse 2-3 times until the meat is crumbled really small. Return to pan. Season with salt and pepper. Add garlic and onions and cook for 2-3 minutes.
Add tomato sauce, tomato paste, diced tomatoes, water and Italian seasoning. Simmer for 15-20 minutes. Taste sauce and more salt and pepper, Italian seasoning or garlic powder, if needed.
Meanwhile, cook the jumbo pasta shells according to package instructions, until al-dente. Drain from water and lay out on a large baking sheet to cool, so that they don't stick together.
Add the ricotta cheese, egg, 1 cup mozzarella, and 1/3 cup parmesan cheese to a large mixing bowl and mix well to combine.
Spread 1 cup of the meat sauce into the bottom of a 9x13'' pan. Stuff each cooked shell with a small spoonful of ricotta mixture, then a spoonful of meat mixture and set them face up in a large baking dish.
Spoon the remaining red sauce over the top of all of the stuffed shells. Sprinkle with remaining mozzarella and parmesan cheese.
Bake at 350 degrees for 25-30 minutes.
Vegetarian Stuffed Shells: omit the meat.Make Ahead Instructions: assemble the entire dish, cover and store in the fridge until ready to bake.Freezing Instructions: Lasagna stuffed shells makes for an excellent freezer meal. Assemble the dish following the recipe directions but don't bake. Consider using a disposable container or aluminum pan. Once the meal is assembled, cover it with a layer of plastic wrap and then a double layer of aluminum foil. Freeze for up to 3 months. Thaw overnight, remove plastic wrap, cover with foil and bake for 45 minutes covered, then 15 minutes uncovered.