"I am a professional chef whose family prefers simple, homey dishes. They nicknamed me "The Diner Queen" because of recipes like this. It takes more time to assemble than a typical casserole that contains the dreaded canned soup. The extra time and effort are definitely worthwhile. This can be made ahead and baked when needed. Bring to room temperature before baking. If a crunchy topping is desired, try sprinkling crushed french fried onion rings on top of the melted cheese and baking for a few extra minutes."
Preheat oven to 350 degrees. Lightly grease a 1-1/2 quart casserole.
Cook noodles in boiling salted water until just done, about 6 minutes. Drain and rinse under cold water. Set aside.
Melt 3 tablespoons butter in a medium saucepan. Add flour and whisk on medium heat until bubbly, about 2 minutes.
Gradually add milk and whisk constantly until mixture just begins to boil. Remove from heat and stir in salt, pepper, Old Bay seasoning, cayenne pepper and Sherry. Add 8 ounces of cheese and stir until melted. Taste and adjust seasoning if desired. Set aside.
In a small skillet saute celery and onion in remaining 2 tablespoons butter. Cook on medium heat, stirring frequently, until limp. Add vegetables to sauce.
Combine cooked noodles and sauce in prepared baking dish. Gently fold in tuna and blend well. Cover with foil.
Bake in preheated oven for 30 minutes. Remove foil and sprinkle remaining 4 ounces of cheese on top. Bake another 15 minutes until bubbly.