Position a rack in the middle of the oven and preheat to 425 degrees.
Bring a large pot of lightly salted water to a boil over high heat. Add the pasta and cook until al dente, according to the package instructions. Drain the pasta and rinse with cold running water.
In a liquid measuring cup, whisk together the milk, garlic and nutmeg.
In a medium (3-quart) saucepan over medium-high heat, melt the butter until foamy. Sprinkle in the flour and cook, whisking constantly, until the mixture is blond or very light brown, about 1 minute.
Slowly whisk in the milk mixture, bring to a simmer and cook, whisking constantly and scraping the bottom of the pot so it doesn’t scorch, until the mixture thickens, 3 to 5 minutes. Remove from the heat.
Add the ’nduja and mustard, and whisk until thoroughly combined. Add the cheddar and whisk until melted. Stir in the cooked pasta.
Transfer to a 9-by-9-by-2-inch or similarly sized baking dish. Dollop the blue cheese evenly over the pasta mixture, then sprinkle the panko evenly over the top. Bake for 15 to 20 minutes, or until the cheese is bubbling around the edges and the top is golden brown. Let cool slightly before serving.
