How to Make It
In a large pot of salted boiling water, cook the pasta until just al dente, about 10 minutes. Drain well. Butter a 9-by-13-inch baking dish.
Meanwhile, in a medium saucepan, melt the butter over moderate heat. Whisk in the flour until a paste forms; continue whisking until the paste is light golden. Whisk in the heavy cream and milk, a little at a time at first to avoid lumps. Bring to a boil and cook, whisking constantly, until smooth and thickened, about 3 minutes. Reduce heat to low and whisk in the cream cheese, Grana Padano and 1 cup of the cheddar. Season with 1 teaspoon salt.
Preheat the broiler. In a bowl, toss the pasta with the hot cheese sauce. Transfer to the buttered dish. Top evenly with the remaining 1 cup of cheddar and the breadcrumbs; season with salt and pepper. Broil 6 inches from the heat for 4 to 6 minutes, until the crumbs are browned and the cheese sauce is bubbling. Top with the scallions and serve.