01 - Preheat your oven to 375°F (190°C). Lightly grease a baking dish.
02 - Bring a large pot of salted water to a rolling boil. Cook the elbow macaroni until it reaches an al dente texture, following package directions. Drain the macaroni and set aside.
03 - While the pasta is cooking, prepare the bell peppers. Slice off their tops, then remove the seeds and internal membranes. Stand the hollowed peppers upright in the prepared baking dish.
04 - In a medium saucepan, melt the unsalted butter over medium heat. Whisk in the all-purpose flour and cook for 1 minute, stirring continuously.
05 - Gradually whisk in the whole milk, salt, black pepper, Dijon mustard (if using), and smoked paprika. Continue whisking and cook until the mixture has slightly thickened, approximately 2–3 minutes.
06 - Remove the saucepan from the heat. Stir in the shredded sharp cheddar cheese and shredded mozzarella cheese until they are fully melted and the sauce is smooth.
07 - Gently fold the cooked elbow macaroni into the cheese sauce, ensuring all pasta pieces are thoroughly coated.
08 - Spoon the prepared macaroni and cheese mixture into each bell pepper, filling them generously to the brim.
09 - In a small mixing bowl, combine the grated Parmesan cheese, panko breadcrumbs, and olive oil. Sprinkle this mixture evenly over the top of the stuffed peppers.
10 - Cover the baking dish with aluminum foil and bake for 25 minutes. Subsequently, remove the foil and continue baking for an additional 10 minutes, or until the tops are golden brown and the peppers are tender.
11 - Allow the stuffed peppers to cool for 5 minutes before serving.