Prepare the Peppers:

1. Preheat the oven: Set your oven to 375°F (190°C). This moderate temperature cooks the peppers until tender without burning the cheese topping.
2. Prep the peppers: Cut each bell pepper in half lengthwise, cutting right through the stem (you can leave it on for presentation, or remove it). Use a spoon to scoop out all the seeds and white membranes. Lightly brush the outside of each pepper half with olive oil. This helps them roast beautifully and prevents sticking.
3. Arrange in baking dish: Pour the chicken or beef broth into the bottom of a 9×13-inch baking dish. Arrange the pepper halves cut-side up in the dish. The peppers can nestle closely together. Set aside while you make the filling.

Make the Sausage Filling:

4. Brown the sausage: Heat a large skillet over medium-high heat. Add the Italian sausage and break it up with a wooden spoon or spatula. Cook for 6-8 minutes, stirring occasionally and breaking the meat into small crumbles, until browned and cooked through. The sausage should no longer be pink and should develop some nice brown bits.
5. Add aromatics: Reduce heat to medium. Add the diced onion to the sausage and cook for 4-5 minutes until softened and translucent. Add the minced garlic and cook for another 30-60 seconds until fragrant. Be careful not to burn the garlic.
6. Combine the filling: Transfer the sausage mixture to a large mixing bowl. Add the cooked rice, diced tomatoes (drained slightly), 1 cup of the shredded mozzarella cheese (saving the rest for topping), Italian seasoning, salt, pepper, and red pepper flakes if using. Stir everything together until well combined. Taste and adjust seasoning if needed – the filling should be well-seasoned and flavorful.

Stuff and Bake:

7. Fill the peppers: Using a large spoon or ice cream scoop, generously fill each pepper half with the sausage and rice mixture. Pack it in nicely and mound it slightly on top. You want each pepper nice and full. Divide all the filling evenly among the pepper halves.
8. First bake: Cover the baking dish tightly with aluminum foil. Bake in the preheated oven for 35 minutes. The foil traps steam, which helps soften the peppers while keeping the filling moist.
9. Add cheese and finish: After 35 minutes, carefully remove the foil (watch out for steam!). Sprinkle the remaining ½ cup of mozzarella cheese evenly over the tops of the stuffed peppers. Return to the oven, uncovered, and bake for an additional 5-10 minutes until the cheese is melted, bubbly, and slightly golden around the edges.
10. Rest and serve: Remove from the oven and let the stuffed peppers rest for 5 minutes before serving. This allows the filling to set slightly and makes them easier to serve. Garnish with fresh chopped parsley or basil and a sprinkle of Parmesan cheese if desired. Serve hot and enjoy!