Preheat oven to 450°F Grease 4 standard size muffin cups ( 2 1/2 inch diameter ) of a 6 cup muffin tin.
Sift flour and salt into a bowl. In separate bowl beat egg, milk. butter and mustard. Pour over dry ingredients and stir to form a batter. Do not beat mixture!.
Fill bottom of each muffin cup with 1 Tbs of the cheese to evenly distribute over bottom of muffin cup. . Divide batter among the muffin cups and sprinkle top of batter with remaining Tbs of the cheese. Fill the empty muffin tins 1/2 way with water. Bake 15 to 20 minutes or until puffed and golden. Carefully remove from oven. Remove from muffin tin and serve warm.