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.
In a large saute pan or skillet, heat the oil over medium-high heat. Add the onions and chopped bell peppers and cook, stirring, until soft, about 3 minutes. Add the beef, pork, garlic, parsley, salt, black pepper, and pepper flakes. Cook until the meat is browned, stirring with a heavy wooden spoon to break up the lumps, about 6 minutes. Add the rice and tomato sauce 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 in the baking dish. Bake until the peppers are very tender and the filling is heated through, 25 to 30 minutes.
Remove from the oven and let rest for 10 minutes before serving.