Preheat the oven to 425.
In medium sized pot, over medium heat, melt butter.
Add the flour and cook for 3 minutes.
Pour in the hot milk and cook until thickened.
Add the spinach, cheese, seasonings, horseradish and mustard.
Mix well.
Combine with the cooked pasta.
Place in a greased cassarole dish.
Bake in oven for 15 minutes or until bubbling.
