01 - Set oven to 375°F (190°C). Lightly grease a baking dish suitable for upright peppers.
02 - Boil salted water in a large pot. Cook macaroni until al dente, approximately 7-8 minutes. Drain and set aside.
03 - Melt butter in a medium saucepan over medium heat. Whisk in flour and cook for 1 minute, stirring constantly.
04 - Gradually whisk in milk to prevent lumps. Continue cooking and stirring until the sauce achieves a thickened consistency, about 3-4 minutes.
05 - Remove the saucepan from heat. Stir in cheddar, mozzarella, Dijon mustard, paprika, salt, and pepper until the cheese is fully melted and the sauce is smooth.
06 - Add the cooked macaroni to the cheese sauce and mix thoroughly to ensure even coating.
07 - Arrange the prepared bell peppers upright in the prepared baking dish. Fill each pepper generously with the macaroni and cheese mixture.
08 - In a small bowl, combine panko breadcrumbs, melted butter, and Parmesan cheese. Distribute this mixture evenly over the stuffed peppers.
09 - Cover the baking dish loosely with foil and bake for 25 minutes. Then, remove the foil and continue baking for an additional 10 minutes, or until the tops are golden brown and the peppers are tender.
10 - If desired, garnish with chopped fresh parsley. Serve hot.