Bring a large pot of salted water to a boil. Add the pasta and cook as the label directs, adding the broccoli during the last 3 minutes of cooking. Reserve 1 cup cooking water, then drain.
Meanwhile, melt the butter in a large saucepan over medium heat. Add the Canadian bacon and cook, stirring occasionally, until golden and slightly crisp, about 5 minutes. Transfer to a paper towel-lined plate using a slotted spoon; reserve the drippings in the pan.
Add the scallion whites, 1/4 teaspoon salt and a few grinds of pepper to the saucepan. Cook until slightly softened, 1 minute. Whisk in the flour and mustard; cook, whisking, until the flour is lightly toasted, about 2 minutes. Whisk in the milk and reserved pasta water. Cook, whisking, until slightly thickened, about 5 minutes. Add the cheeses; cook, whisking, until creamy, about 2 more minutes.
Stir the pasta, broccoli and Canadian bacon into the cheese sauce. Season with salt and pepper. Serve topped with the scallion greens.
Photograph by Charles Masters
Courtesy of Food Network Magazine