Preheat the oven to 160˚C.
In a large bowl, combine the crème fraiche, egg yolk, garlic, onion, cheddar and mozzarella.
Stir in the shredded chicken and mash potatoes. Season well with salt and pepper.
Transfer the mixture in a gratin dish.
In a second bowl, combine the breadcrumb, herbs and cheese leftover. Season well.
Sprinkle the mixture over the mash potato gratin.
Place in the oven and bake for 30-45 minutes until piping hot and golden brown on the top.
Remove from the oven and serve immediately.