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 (1/2 pound), Italian sausage (1/2 pound), garlic (1 1/2 tablespoons), parsley (1/4 cup), salt (2-3 teaspoons), black pepper (1 teaspoon), and pepper flakes (1/4 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 1/8-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.
Notes
Parboil: When considering how to make stuffed peppers recipe, it helps to parboil the peppers first. It only takes a couple of minutes and ensures these peppers come out of the oven nice-n-tender. You’ll also want to let them dry well for a deliciously cohesive dish. If you skip this step your peppers may stay too crunchy.
Cook Time: You’ll want to take the size of your bell peppers into account when deciding how long to cook stuffed peppers recipe. Most often it will be in the 25-30 minute range.