For the topping, add the toasted pecans to a food processor and pulse until they look like bread crumbs. Add the butter to a medium skillet and melt over low heat; once melted, continue cooking over low heat until the butter turns golden brown, about 5 minutes. Add the Panko, salt, and black pepper, and cook until the Panko is browned, about 5 minutes, stirring occasionally. Stir in the pecans and sage, and turn off the heat. Set aside.
For the mac and cheese, cook the pasta to al dente according to the package directions; drain and set aside.
Preheat the oven to 400F; grease a 9 by 13-inch casserole dish with butter.
Add 2 tablespoons butter to a large saucepan over medium heat; once hot, add the onion and cook until softened but not browned, about 5 minutes, stirring occasionally. Add the garlic and flour and cook 1 minute more, stirring frequently.
Slowly whisk in the milk until lump-free, then whisk in the pumpkin, Worcestershire, Dijon, bay leaf, sweet paprika, salt, black pepper, and hot sauce (if using). Bring to a simmer and then cook for 2 minutes, stirring constantly. Stir in the brie and cheddar until completely combined.
Stir in the pasta and sage and pour the mac and cheese into the prepared dish; sprinkle the topping evenly on top.
Bake until the casserole is bubbling, about 10 minutes.
Let stand 5 to 10 minutes before serving.