Preheat the oven to 375°F. If making rice the day of, do this first. Prepare it according to package directions and set aside. Heat a large skillet over medium heat with 1 tablespoon of olive oil.
Once the skillet is hot and the oil is glistening, add in the onions and red bell pepper with a pinch of salt. Cook for about 4-5 minutes, until the peppers and onions are beginning to soften. Stir occasionally to prevent burning.
Next, add in the garlic, 3 tablespoons of tomato paste, red pepper flakes to taste, and 1 tablespoon of Italian seasoning, stirring often to prevent the garlic from burning. Cook until the tomato paste has deepened in color, about 2-3 minutes.
Add 1-2 tablespoons more olive oil as needed, followed by the Italian sausage. Cook and break up the sausage into small pieces, stirring occasionally until the meat is completely cooked through.
Add in the can of diced tomatoes, followed by ½ - 1 cup of chicken broth as needed, and 1 cup of cooked white rice. Simmer until the liquid is mostly absorbed, about 5-6 minutes, then add in fresh basil if desired and stir to wilt. This should take about 1 minute.
Add in about half (3 ounces) of the shredded mozzarella and (2 ounces) parmesan, and remove the skillet from the heat. Stir to melt the cheese. At this point, adjust seasonings to ensure they are to your liking.
Add the bell peppers to a baking dish upright, and sprinkle the insides with just a little bit of salt. Spoon them full of the filling. Sprinkle the remaining cheese mozzarella cheese (3 ounces) and parmesan cheese (2 ounces) over top of the peppers.
Bake for 35-40 minutes, or until the peppers can be pierced with a fork, and they are softened to your liking. The cheese should be golden brown on top. If desired, you can broil on high for 2-3 minutes to get the cheese nice and crispy.
Garnish with more freshly grated parmesan cheese, and basil before digging in. Enjoy!
Store leftovers in an airtight container in the fridge for up to 4-5 days. Reheat gently in the microwave, oven, or air fryer. You can also freeze leftovers for up to 2 months, though it is better to freeze the peppers before baking if possible, to help the cheese maintain proper texture.