Place six of your eggs in a pot of water and cook them until they are hard boiled, about 8 minutes. Once they are ready, remove the shell, slice and set aside.
Gently melt the butter in a frying pan on a medium heat. Add the onion and sauté until soft.
Stir in the flour. Slowly add the milk and cook until thick, stirring constantly.
Next, add the cheese, salt, pepper, mustard and fold in your eggs.
Place in the fridge for about 2 hours.
On a flat surface roll the mixture into 12 croquettes.
Gently beat your two leftover eggs in one bowl and pour your breadcrumbs into another.
Dip your croquettes into the egg and then the breadcrumbs.
In a large frying pan, heat the oil on a medium to high temperature. Gently fry the croquettes in the oil for about 2 minutes or until brown.