Place the flour, dry mustard and pepper in the saucepan. Gradually add the milk, stirring with a whisk until blended. Cook over medium heat until thick (5 to 8 minutes). Add the cheeses, stirring frequently, until melted. Turn off the heat and add the spinach, salt and the pasta and mix together.
Put the mixture in your casserole dish that you have sprayed with cooking spray. Sprinkle the bread crumbs or panko over the mixture.Bake at 375 degrees for 30 minutes, or until bubbling.
Notes
I don't use processed cheese. I think you can get a smooth creamy sauce without it.