Heat the oven to 350 F and spray a medium casserole dish with non-stick cooking spray. Cook the pasta in salted water according to the package directions, until slightly less than al dente. Drain and rinse under cool water; set aside.
In a medium sauce pan, melt the butter over medium heat until it foams. Add the grated onion, salt, pepper, mustard powder, and cayenne pepper and cook until fragrant, about one minute.
Make a roux by adding the flour and whisk until smooth; whisking constantly, until the mixture is a light golden brown. Continue whisking; add the beer and half and half and cook until smooth; let simmer until thickened; add the cream cheese and whisk until completely smooth
Reduce heat to low; while stirring, add the cheese, reserving ¾ cup of the Parmesan for topping the casserole; continue stirring. Once smooth turn off the heat and fold in the cooked pasta.
Turn the mixture out into the prepared casserole pan and sprinkle with the reserved Parmesan cheese; sprinkle the crushed Pretzel Pieces over the top of the casserole.
Bake for 40 to 45 minutes or until the topping is golden brown.