Butterfly the chicken breast and open it out like a book. If necessary, flatten the meat further by pounding with a rolling pin, but don't split the meat.
Beat the egg in a bowl, dip the meat then coat with breadcrumbs. Cover and place in fridge while preparing the sauce.
Place the milk in a heatproof jug, add the cardamoms, cloves, peppercorns, onion and bay leaf. Place in microwave and heat at full power 1-3 minutes, taking care it does not boil.
Melt the butter in a saucepan, add the flour and stir constantly to form a roux. Remove from heat. Strain the heated milk into another jug to remove the spices.
Place the roux back on the heat and slowly add the hot milk, stirring rapidly to prevent lumps. If it gets too thick add more milk, but keep stirring. Add the Parmesan and remove from heat. It will thicken as it cools.
Place the chicken breast under a preheated grill to cook, turning once once side is done. Smother the other side of the breast with the sauce and top with grated Cheddar. Return to the grill and cook until sauce is bubbling and cheese has melted.