Preheat the oven to 350 degrees F.
For the macaroni: Bring a large pot of salted water to a boil. Cook the macaroni to al dente according to the package instructions. Drain and set aside.
For the sauce: In a large saucepot on medium heat, add the butter, onion, Worcestershire sauce, flour, paprika and a few pinches of salt as well as black pepper. Cook for a few minutes while stirring, then add the half-and-half and cream cheese. Cook until the mixture begins to simmer.
For the dish: Butter the sides and bottom of a 13-by-9-inch casserole dish or lasagna pan. Mix both cheese blends together in a large bowl.
Put the scallions and cooked macaroni in a large bowl and toss in 4 cups of the shredded cheese. Pour over the sauce and stir, shake, jiggle and tap the bottom of the bowl lightly to make sure the sauce seeps completely into the nooks and crannies. Pour this mixture into the prepared dish. Toss the fried onions with the remaining 2 cups of cheese and sprinkle over the top. Bake until the top is melted and bubbly, 20 to 25 minutes.
Have less time? You don't need to bake this if you don't have time: just serve it after stirring in the scallions, pasta and cheese sauce.