Cook pasta in large pan of boiling water, uncovered, until just tender; drain.
Heat butter in large pan, add flour; cook, stirring, until flour thickens and bubbles.
Remove from heat; gradually stir in 3 cups of the milk (if recipe is to be frozen, add the remaining cup of milk).
Cook, stirring, until mixture boils and thickens; stir in cheddar cheese, mustard, ham, corn and onion.
Gently stir pasta into pan with sauce; spoon into 2 1/2-litre (10-cup) lightly oiled baking dish, [can be prepared ahead to this point; refrigerate overnight or freeze] Sprinkle with pizza cheese; bake, uncovered, in moderately hot oven about 20 minutes or until top is browned.