Put the dry macaroni elbows in a suitable pan - (yes, dry macaroni, no need to pre cook).
Melt the butter, and then pour it over the macaroni.
Add both cheeses.
Add milk.
Add heavy whipping cream.
Add salt, pepper and mustard
Give it a good stir.
Cook without a lid on a smoker set at 250° F for 1hr 50mins.
Notes
For a crunchy top, melt a tablespoon of butter in a saucepan and mix in 1/2 cup of panko crumbs, crushed corn flakes, or breadcrumbs. Cook until the crumbs are golden, then set aside. Sprinkle these over the macaroni and cheese in the last half-hour of cooking.
Can be cooked in a slow cooker, but will take longer - 2-3 hours on High. If using the crunchy top, leave pan uncovered for last half hour of cooking.