Preheat oven to 350°F. Cook pasta to al dente according to package directions; drain well in colander and rinse with cold water; drain well and shake to remove excess water. Set aside.
Melt 1/2 cup butter in 4-quart, heavy-bottomed saucepan over medium heat. Whisk in 1/4 cup flour and salt. Slowly add milk, whisking constantly to prevent clumping. When sauce is smooth and has thickened, remove from heat. Immediately add 3 cups cheddar cheese to sauce; whisk until cheese melts completely and sauce is smooth.
Add cooked pasta, chicken and vegetable mix to sauce; mix well. Pour mixture into buttered 7 x 11-inch baking dish; smooth surface with spatula.
To make topping, combine remaining 2 tablespoons butter, 3/4 cup flour, 1 cup cheese and pepper in medium bowl. Using fingertips, mix together until mixture resembles coarse meal. Sprinkle topping over surface of macaroni mixture.
Bake casserole, uncovered, 25-30 minutes, or until topping becomes light brown and pasta is heated through.