Preheat the oven to 350F and spray a 2 quart casserole or 9"x13" baking dish with non-stick spray.
Start by cooking your pasta in a large pot. Follow instructions on the package, or until al dente. Drain the pasta and reserve.
Place the same (emptied) large pot over medium heat. Melt the butter in the pot. Add the chopped onion and minced garlic and cook for a few minutes.
Sprinkle the ¼ Cup of flour over the onion and garlic and stir for the flour to be absorbed by the butter. Cook for a couple minutes. Then proceed with slowly whisking in the milk while stirring constantly to keep it smooth until thickened.
Once the sauce has thickened, turn the heat off. Add in the cream cheese in small cubes and stir until melted. Continue to add the mustard, buffalo sauce, canned chicken, chopped bell peppers, grated cheese, and cooked pasta.
Stir everything together until the cheese is melted and the pasta is coated in the sauce. Pour the mac and cheese into the prepared dish.
Combine the panko crumbs, parmesan cheese, and chopped parsley together in a small bowl and stir to combine. Evenly sprinkle over the mac and cheese to cover.
Place the baking dish in the preheated oven and bake for 20 minutes. Take out of the oven and sprinkle with the cooked bacon bits. Let rest for 5-10 minutes and serve hot!