Cook macaroni according to package direction.
When macaroni is half way cooked add frozen vegetables and continue cooking until macaroni is tender.
Drain through a colander and keep warm.
In sauce pan, add 1 1/2 cups of the milk to the pot and bring to a simmer.
Whisk together remaining milk, flour, salt and pepper.
Slowly add flour mixture to simmering milk, simmer until thickened.
Add both cheeses and mustard, stir constantly until cheese is melted completely.
Add Macaroni and Vegetables, stirring until well coated.