01 - Preheat oven to 400°F (200°C). Lightly grease a large baking dish.
02 - Boil a large pot of salted water. Cook pasta according to package directions, adding broccoli florets during the final 3 minutes of cooking. Drain thoroughly and set aside.
03 - In a medium saucepan, melt butter over medium heat. Add the chopped onion and cook until softened, approximately 3 minutes. Stir in the minced garlic and continue cooking for 1 minute until fragrant.
04 - Sprinkle the all-purpose flour over the sautéed aromatics. Stir continuously for 1-2 minutes until the mixture turns lightly golden, forming a roux.
05 - Gradually incorporate the whole milk into the roux, whisking constantly to prevent lumps. Increase heat to bring the mixture to a gentle simmer, continuing to whisk until the sauce has thickened, about 3-5 minutes.
06 - Remove the saucepan from the heat. Stir in the grated cheddar cheese, mozzarella cheese, Dijon mustard, paprika, salt, and pepper until the cheeses are fully melted and the sauce is smooth and creamy.
07 - Gently fold the cooked pasta and broccoli into the cheese sauce until evenly coated. Transfer the mixture to the prepared baking dish.
08 - In a small bowl, combine the panko breadcrumbs, grated Parmesan cheese, and olive oil. Mix until well combined.
09 - Evenly sprinkle the breadcrumb mixture over the pasta in the baking dish. Bake for 20 minutes, or until the top is golden brown and the sauce is bubbling. For enhanced crispiness, you may broil for an additional 2-3 minutes at the end, watching carefully.
10 - Allow the bake to cool slightly before serving hot. For a festive presentation, consider garnishing with edible Halloween shapes such as olive spiders or pepper jack-o'-lantern faces.