Remove crust from bread and cut into 1" cubes. Set aside. Melt the butter over low heat. Stir in cheeses, mustard, pepper and salt until well blended. Fold in stiffly beaten egg whites. Spear bread cubes (or use a fork to toss) and dip into cheese mixture coating well. Use a knife to scrape off any extra or hanging cheese that is too much on the bottom of the bread cube. Put these on a cookie sheet you have covered with either wax paper or parchment paper. Freeze until firm and then place in freezer bags until ready to serve.