Preheat oven to 350 degrees F. Spray an 8 x 8 inch or 2 1/2 quart baking dish with nonstick cooking spray.
Cook macaroni and frozen vegetables in boiling water until partially cooked, approximately 7 minutes. Drain well.
In a large skillet, brown onions in butter until softened.
Whisk in flour, dry mustard, salt and pepper.
Slowly add in milk, bringing it to a slow simmer until thickened.
Stir in tuna and cooked macaroni and vegetables.
Pour into prepared dish.
Top with thin slices of Velveeta or cheese melt.
Cook for approximately 20 minutes or until hot and bubbly. The Velveeta can be smeared across the entire casserole once it starts melting, approximately 12 minutes into the baking time.