Stuffed Peppers, Chef John Style!

"This recipe's roots can be traced back to the Great Depression, when farmers first began selling the unripe peppers out of necessity. Of course, we did what Americans always do when faced with a new variety of food, we stuffed meat in it." ~Website link for recipe

Ingredients for the sauce:
1 tablespoon olive oil
1 onion, diced
1 cup beef broth
2 cups prepared marinara sauce, or other tomato sauce
1 tablespoon balsamic vinegar
1/4 tsp red pepper flakes, optional

For the peppers:
1 pound lean ground beef
1/4 pound hot Italian pork sausage, casings removed
2 cups cooked rice
1 cup finely grated "real" Parmesan cheese (Parmigiano Reggiano)
1/4 cup chopped Italian parsley
4 cloves garlic, minced very fine
1 can 10-oz diced tomatoes
2 tsp salt
1 tsp freshly ground black pepper
pinch of cayenne
4 large green bell peppers