On medium-low heat, saute the celery in the butter for 15 minutes, stirring often. The celery should become somewhat translucent and shrink slightly. Add the milk and bring to barely simmering; stir in the cornstarch and water mixture. This should thicken it slightly. Stir, not letting it go above a simmer, for 30 seconds.
Stir in the cheddar cheese. Stir in the macaroni and tuna until well mixed. Season with salt and pepper as desired.
Pour into a casserole dish and bake for 15-20 minutes. Remove from oven, sprinkle on the french-fried onions, and cook for another 5 minutes.