Ingredients
Melt butter in a saucepan.
Whisk in flour and cook 1 minute.
Slowly whisk in milk and cook until thickened.
Remove from heat and stir in cheeses until smooth.
In a small bowl, whisk egg yolk with a spoonful of the hot sauce, then whisk back into the pan.
Stir in mustard, lemon juice, salt, and cayenne.