Heat your oven to 350˚F.
In a sauce pan melt half the butter over medium heat. Stir in the flour, then whisk in the milk and add the mustard, celery salt, kosher salt, and pepper. Bring to a simmer, then add half the cheese and stir until melted.
Add the cooked pasta to the pan and stir till everything is nicely combined.
In a small skillet, melt the remaining butter and add the bread crumbs and cook until lightly browned, a few minutes.
Pour the pasta into a baking dish (Ann always butters her dish first). Cover it with the breadcrumbs and dot the surface with the remaining chunks of cheese.
Bake the pasta for 30 minutes or until bubbly and golden brown on top. (It's a good idea, depending on how full your dish is, to cover a baking sheet with foil or parchment and put the baking dish on this in case it bubbles over.)