01 - Preheat your oven to 200°C (400°F). Lightly grease a large baking dish with approximate dimensions of 23x33 cm (9x13 in).
02 - Bring a large pot of generously salted water to a rolling boil. Add the pasta and cook for 2 minutes less than indicated on the package instructions. During the final 2 minutes of cooking, add the broccoli florets to the boiling water. Drain the pasta and broccoli thoroughly and set aside.
03 - In a large saucepan, melt the butter over medium heat. Add the finely diced onion and sauté for 2 minutes until it begins to soften. Stir in the minced garlic and cook for an additional 1 minute until fragrant.
04 - Incorporate the all-purpose flour into the saucepan with the sautéed aromatics. Cook, stirring constantly, for 1 minute to form a roux. Gradually whisk in the whole milk, ensuring no lumps form. Bring the mixture to a gentle simmer, continuing to whisk until the sauce thickens, which should take approximately 4–5 minutes.
05 - Remove the saucepan from the heat. Stir in the shredded cheddar cheese, grated Parmesan cheese, ground nutmeg, salt, and black pepper until the sauce is completely smooth and well-combined.
06 - Gently combine the drained pasta and broccoli with the prepared cheese sauce in a large bowl. Pour this mixture evenly into the prepared baking dish.
07 - To create the Halloween theme, slice the pitted black olives in half lengthwise and arrange them on top of the pasta bake to resemble spiders. Cut the orange bell pepper into small shapes for festive faces (smiley or jack-o'-lantern) and place them artistically on the surface.
08 - Evenly sprinkle the shredded mozzarella cheese and panko breadcrumbs (if using) over the entire surface of the bake. Place the dish in the preheated oven and bake for 20–25 minutes, or until the topping is golden brown and the bake is bubbling.
09 - Allow the pasta bake to cool for approximately 5 minutes before serving. This resting period helps the bake set and makes it easier to portion.