Melt butter in heavy saucepan on top of stove.

Add flour, stir and let cook for 1 minute, being careful not to scorch.

Slowly add milk, whisking to combine. Stir constantly until mixture comes to a boil. Cook and stir 1 minute longer.

Remove pot from heat and stir in cheese and mustard until melted.

Toast English muffins and top with chicken, sliced tomatoes, and the cheese sauce. Broil for 3-4 minutes, watching closely until tops are browned and bubbly.