How to Make It
Cook pasta shells according to package directions; drain well.
In a large pot, melt butter over medium heat; stir in the flour to form a thick paste. Whisk in the milk, heavy cream, and Dijon mustard, whisking until combined.
Add the white cheddar cheese, white American cheese, and salt to pan; stir until melted and combined. Stir in cooked pasta shells. Serve immediately.