Preheat oven to 350*
Place large skillet over medium high heat and brown ground beef. Using your wooden spoon, break up the beef into small pieces.
While the beef is browning, chop the onion and the mushroom into small dice. As the beef is starting to brown, move it to the side and add the onion and mushroom. Stir and mix all the contents of the skillet until soft and translucent.
Sprinkle mixture with 1 tsp of salt and and ½ tsp pepper and stir to combine. Add in the cream cheese and the parmesan cheese to the meat mixture. Stir to melt and combine.
Pour in 1/2 cup marinara over the beef mixture. Stirring to combine all. Taste for your desired seasoning.
While beef is cooking, cut all your peppers in half and scoop out all seeds and ribs. Cut from the top down through the stem. This ensures that you have a boat shape for your filling to sit.
Drizzle 1-2 tbsp olive oil into the casserole dish. Place all your pepper boats snuggly into the casserole dish on top of the marinara sauce. I can usually get 6 halves in one regular sized casserole dish.
Spoon the meat mixture into each of the peppers equally. Sprinkling with shredded mozzarella cheese on top.
Cover your casserole dish tightly with tin foil and place into a 350* oven. Bake for 40 minutes. Then take foil top off and broil for 3-5 minutes on high.