Fill a large stock pot with water and bring to a boil.
While the water is heating begin cleaning and hollowing out the bell peppers. Once the water is to a boil carefully place each bell pepper in the boiling water and boil for 10 minutes. *You want your peppers to be firm not mushy. Remove the peppers from the boiling pot of water and rinse under cold water. Set aside.
Preheat the oven to 400 degrees.
In a large skillet begin browning the lean ground beef on medium heat (for about 5 minutes). Half way through the browning process as your breaking up the bits of ground beef add the diced onion, garlic, salt, pepper, and garlic powder.
Once the ground beef is cooked through turn off the heat. Add the rice, shredded zucchini (if using) and 3 cups of tomato sauce, and Italian seasoning. Mix well until incorporated.
Carefully begin spooning the rice/beef mixture into each of the bell peppers until the pepper is packed full. *You may have filling leftover depending on how large your peppers are. Reserve for serving once peppers are done.
Add 1 cup of water to the baking dish.
Tent a piece of aluminum foil over the baking dish and bake for 20 minutes.
Remove the peppers from the oven. Add more sauce on top and sprinkle on the shredded mozzarella cheese. Bake uncovered for 5 minutes or until cheese has melted.
Remove from oven, garnish with fresh minced parsley and additional sauce if needed, and serve immediately.