Preheat oven to 180 degrees C. Coat a 2 litre casserole dish with cooking spray.
In a large pot of salted water, boil noodles until al dente. Drain well.
In a medium saucepan, combine flour, butter and salt. Stir until butter is melted and ingredients are combined evenly. Add milk, and whisk until the sauce thickens (usually it is at the proper consistency by the time it begins to boil). Add cheese to mixture, and whisk until cheese is melted and mixture is well blended. Stir in tuna, peas and noodles. Spread evenly in prepared dish.