Preheat the oven to 350 F
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 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 heat and adjust the seasoning, to taste.
Pour enough water (or chicken broth) into a baking dish just to 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 in heated through, approximately 20 minutes.
Remove from the oven, sprinkle with mozzarella cheese and bake for an additional 5 to 10 minutes.
Relevant Fahrenheit temperatures for Cooking, Aging and Preserving Beef
Below 32° - microbe multiplication largely ceases but does not die and will resume when temperatures rise. Any toxins previously released will still be dangerous when meat thaws. Enzyme activity transforming meat is dormant.
36° to 39° - temperature range for safe dry aging
40° to 140° - temperature range for rapid harmful and beneficial microbial growth -- 141° to 165° temperatures lethal to most microbes - check with digital thermometer and cook all meat to 165° for food safety, particularly Chicken, Turkey and other fowl.
150° to 160° - temperatures in which enzyme activity that melts collagen begins to 180° collagen melting ceases