Cut 1/2" slice from stem end of each tomato. Scoop out seed and core. Sprinkle inside with salt & pepper.Turn open end down on paper towels to drain. in a small saucepan, melt butter. Stir in flour. Cook and stir over low heat until bubbly. Stir in the milk; cook and stir over low heat until thickened. Stir in mustard, cheddar cheese and chives. Continue stirring over low heat until cheese melts; do not boil. Set aside. Preheat oven to 400¼F. Grease a 13" x 9"baking pan; set aside. In a large bowl, beat the egg whites until stiff but not dry. Gradually fold in hot cheese sauce. Spoon into drained tomatoes. Arrange filled tomatoes on prepared baking sheet. Bake 15 to 18 minutes in preheated oven until filling is golden brown. Serve at once. Makes 8 servings.