Dice the cooked chicken into medium-sized (3/4-inch) chunks.
Cook the pasta al dente according to the package instructions
Finely dice the onion. Over medium-low heat, cook the onion with butter in a saucepan until translucent.
To the saucepan, add flour and cook for another minute while whisking. Then, add milk and cheese to the pan and stir until the cheese melts and the sauce thickens.
Prepare the breadcrumb mixture by mixing breadcrumbs, finely chopped parsley, and oil in a bowl.
In a 9x13 inch casserole dish, add cooked pasta, diced chicken, and crumbled blue cheese. Toss to combine. Then, mix in the cheese sauce and buffalo sauce. Top with the breadcrumb mixture. Bake in a preheated 350° F oven for 20 minutes, until bubbling with golden-brown breadcrumbs.
Garnish with chopped scallions and drizzle extra buffalo sauce before serving (optional).