Flatten chicken breasts with a wooden mallet and divide into half. Set aside.
Combine salt, mustard and pepper.
Rub on chicken meat and set aside.
In a saucepan, melt butter over moderate heat.
Sauté onions until almost brown.
Add mushrooms and ham.
Stir-fry for at least 30 seconds, lower flame and add grated cheese and milk.
Let cheese melt, while stirring to prevent scorching.
When done, transfer to a flat dish and cool.
Divide cheese mixture into 12 portions.
Line chicken breast flat in a plate and place cheese mixture at the center.
Roll carefully, tucking the edges in.
Roll in flour then, carefully dip in egg.
Roll in breadcrumbs and then fry in moderately hot oil.
When done, drain oil with paper towel.
Happy cooking, Foodies!