Bring a large pot of salted water to a boil. Cook the macaroni until al dente, about 7 minutes; strain and set aside.
Preheat the oven to 350 degrees F. Butter a large oval casserole dish.
Cook 2 tablespoons of the butter, the onions, celery and a pinch of salt in a large saute pan over medium heat until the onions and celery are soft but not brown, about 7 minutes.
Melt 2 tablespoons of the butter in a large straight-sided skillet over medium heat. Sprinkle the flour over the butter and cook for 1 minute. Whisk in the half-and-half, bring to a boil and cook for 2 minutes. Remove the skillet from the heat, and stir in the Cheddar, Monterey Jack and blue cheese until melted. Stir in the onion mixture, macaroni and hot sauce. Taste and season with salt and pepper.
Pour the macaroni mixture into the prepared casserole dish. Melt the remaining 2 tablespoons of butter, pour into a small bowl and mix with the panko; sprinkle on top of the mac 'n' cheese. Bake until bubbly and brown, 35 to 40 minutes. Garnish with the scallions.