Preheat oven to 350°F.
Cut the tops off the peppers removing the interior seeds and ribs. If necessary, cut a thin slice from the bottom of each pepper so they stand upright.
In a large pot of boiling water, parboil the peppers until just tender, 2 to 3 minutes. Remove with a slotted spoon and dry on paper towels.
Arrange standing peppers in a baking dish. Set aside.
In a large skillet, brown the ground beef, until there is no longer any pink, then remove to a paper towel-lined plate; set aside. Drain off excess fat from the skillet.
To the same skillet, add the Italian sausage, onion, and garlic and cook until the sausage is cooked through and the onion is soft. Drain off excess fat from the skillet.
Add the browned beef back to the skillet. Then add the cooked rice, 2 cups shredded Mozzarella, Parmesan, 1/2 cup marinara sauce, egg, Italian seasoning, salt, and pepper. Mix until evenly combined.
Divide the meat mixture evenly among the prepared peppers. Lightly press the mixture as you fill.
Spoon 2 tablespoons of Marinara over the tops of each stuffed pepper. Pour marinara sauce around the peppers into the baking dish just cover the bottom, about 1/2-inch deep.
Bake until the peppers are very tender and the filling is heated through for 25 to 30 minutes.
Remove from oven. Top each pepper with shredded Mozzarella then Italian bread crumbs.
Turn oven to broil. Place peppers, uncovered under the broiler. Broil until the cheese is melted and bubbly, about 5 minutes.
Transfer the stuffed peppers to a platter, spoon the sauce from the baking dish over and around the peppers. Garnish with parsley and serve immediately.