Preheat the oven to 350 degrees F (175 degrees C).
Melt the butter in a small skillet over low heat. Stir in the flour until smooth and continue to cook and stir for 1 minute. Remove the pan from the heat and gradually stir in the milk so that no lumps form. Return to the burner and heat, stirring constantly, until thickened, about 3 minutes. Remove from the heat and whisk in the mustard powder and a little more than half of the Cheddar cheese until melted. Season with salt and pepper.
Flake the tuna and stir it into the sauce along with the parsley and chopped egg. Transfer to a 9 inch round cake pan. Mix together the potato chips and remaining Cheddar cheese and sprinkle over the top. Dust very lightly with paprika.
Bake for 20 minutes in the preheated oven, or until the top is golden brown.
Meanwhile, bring a large pot of lightly salted water to a boil. Add fettuccini pasta and cook for 8 minutes, or until tender; drain. Serve tuna Mornay over fettucine.