Prep the peppers: Slice 1/4 inch off the top of the peppers. Remove the core and seeds as best you can. (Pulling it out by hand usually works best.) Line peppers cut side up in a greased baking pan large enough to fit them all. A 9×13-inch dish is a great size for this recipe. A cast iron skillet could work too.
Preheat oven to 375°F (191°C).
For the filling: Heat olive oil and onion in a large skillet over medium heat. Cook for 3-4 minutes until onions are soft, then stir in the sausage, mushrooms, garlic, oregano, parsley, basil, salt, and pepper. Cook and break up the sausage with your spoon/spatula as best you can until the sausage is almost fully cooked through, about 6-7 minutes. Carefully spoon out most of the grease/fat, if there is any. Stir in the chicken broth. Cook for 2 minutes, then remove from heat. Stir in 1 cup of cheese and all of the cooked rice.
Spoon filling into each pepper, filling all the way to the top.
Optional, but helps keep the peppers moist as they bake—pour about 1 inch of warm water into the bottom of the pan around the peppers.
Bake, covered, for 35-38 minutes or until the peppers are nearly fork tender. Remove from the oven, and evenly sprinkle the peppers with the remaining cheese. Return to the oven, uncovered, and bake for 5 more minutes.
Sprinkle with additional parsley or other herb, if desired. Use tongs to carefully remove the peppers from the pan and serve warm.
Cover and store leftovers in the refrigerator for up to 4 days or freeze for up to 3 months. Thaw overnight in the refrigerator, then reheat in the microwave or cover and bake in a 300°F (149°C) oven for 20 minutes or until warm in the center.