Modified from Chef John:
For the sauce:
1 tablespoon olive oil
1 onion, diced
1 cup beef broth
2 cups prepared marinara sauce, or remaining diced tomatoes from 28 oz can (add oregano, thyme, basil)
1 tablespoon balsamic vinegar
1/2 tsp red pepper flakes, optional
For the peppers:
1 pound lean ground turkey (or beef)
1/2 pound uncooked sweet Italian pork sausage (Trader Joes), casings removed
2 cups cooked rice
1 cup finely grated Parmesan cheese
1/4 cup chopped Italian parsley
1 10-oz diced tomatoes (canned)
2 tsp salt
1 tsp freshly ground black pepper
pinch of cayenne
(1/2 sauted onions from above)
4 large red bell peppers
Add the olive oil to a saucepan, and lightly brown the onions with a large pinch of salt over medium-high heat. Remove half and reserve for the stuffing. Stir in the rest of the sauce ingredients and bring to a simmer. Pour the sauce into the bottom of a large deep casserole dish.
Add all the filling ingredients to a mixing bowl, along with the reserved onions, and stir with a fork, or your hands, until the mixture is combined.
Preheat oven to 375 degrees F.
Cut out top of peppers and parboil for 5 minutes, run cool water to stop further cooking. Cut the bell peppers in half lengthwise. Remove the seeds and white membrane from each pepper. Fill each pepper with the stuffing. Place in largest casserole dish and add a little additional grated cheese over the top.
Cover with foil and bake for 45 minutes. Remove foil and bake uncovered for another 20-30 minutes, or until the peppers are very tender. Exact cooking time will depend on size, shape and thickness of the peppers. Best to let rest for 10 minutes before serving. Serve with the sauce spooned over the top.