Brown ground beef with onions, peppers, mushrooms and garlic in a large skillet over medium-high heat. Season with Italian seasoning, and salt and pepper.
Add in cooked rice and half of the jar of tomato sauce to the meat mixture, and stir to fully combine.
Meanwhile, trim off the stem end of the zucchini and cut them in half lengthwise to make the boats. Pour the remaining half of the jar of tomato sauce into a 9 x 13 large baking dish and put the zucchini halves on top. Next, pour the topping beef and rice mixture over the sliced zucchini pieces. Sprinkle the grated mozzarella on top of the zucchini and stuffing mixture.
Cook at 350, for about 40 mins, or until the zucchini is soft when poked with the tip of a sharp knife.