Preheat the oven to 350°F.

In a large pot of boiling water, parboil the peppers (6) until just tender, 2 to 3 minutes. Remove with a slotted spoon and dry on paper towels.

In a large skillet, heat the oil (1 tablespoon) over medium-high heat. When the oil begins to shimmer, add the onions (1 cup) and chopped bell peppers (1 cup) and cook, stirring until soft, about 3-5 minutes.

Add the beef (½ pound), Italian sausage (½ pound), garlic (1 ½ tablespoons), parsley (¼ cup), salt (2-3 teaspoons), black pepper (1 teaspoon), and pepper flakes (¼ teaspoon). Cook until the meat is browned, stirring with a heavy wooden spoon to break up the lumps, about 6 minutes.

Add the rice (2 cups), tomato sauce (8 ounces), and a pinch of sugar and stir well. Remove from the heat and adjust the seasoning, to taste.

Pour enough water into a baking dish to just cover the bottom, about ⅛-inch deep. Stuff the bell peppers with the rice mixture and place them in the baking dish. Sprinkle the tops with cheese (1 cup). Bake until the peppers are very tender and the filling is heated through about 25 to 30 minutes.

Remove from the oven and let rest for 10 minutes before serving.