Preheat the oven to 400º and place a rack in the middle. Grease a shallow 2-quart baking dish. To make the topping, melt the butter and then stir together with the panko, 2 oz Cheddar and 1/4 cup Romano cheese until well combined.
To make the sauce: Melt butter in a heavy medium saucepan over medium-low heat, and then stir in the flour. Cook roux, stirring, 3 minutes, and then slowly whisk in milk. Bring the sauce to a boil, whisking frequently, and then simmer, whisking occasionally, 3-4 minutes. Stir in cheeses, salt, pepper and dry mustard until smooth. Meanwhile, cook macaroni in a pot of boiling salted water until al dente. Reserve 1/2 cup of cooking water and then drain macaroni. Stir together the macaroni, reserved cooking water and cheese sauce. Transfer to the prepared baking dish. Sprinkle the topping evenly over the macaroni and cheese and bake until golden and bubbling, 20-25 minutes.