Meanwhile, make a white sauce. Melt the butter of medium-low heat. Stir in the flour and mustard and cook a couple of minutes, stirring almost constantly. Turn up the heat a bit and slowly whisk in the milk. Bring to a boil, stirring constantly, then reduce and simmer a minute or two until thickened. Stir in the noodles and tuna, then pour into a 2 quart casserole dish (8x8 pan works great).
Mix the melted butter and bread crumbs, then crumble over the casserole. Sprinkle with paprika.
You can make it ahead to this point, cover and refrigerate. Bake it an extra 10-15 minutes to make sure it's heated through.