Arrange flat mushrooms in an ovenproof dish and season with salt and pepper.
Preheat the oven to 200°C.
Mix chopped mushroom stalks, Chinese mushrooms, garlic, melted butter, egg yolk, breadcrumbs, herbs, and nutmeg.
Divide the mushroom filling evenly between the flat mushrooms.
Boil milk with carrot, onion, peppercorns, and herbs, then simmer for 5 minutes and infuse for 10 minutes.
Cook flour and 15g butter for two minutes to make a roux and let it cool.
Strain the milk, bring to a boil, and whisk in the roux until thickened.
Stir mustard and 15g each of cheddar and parmesan into the sauce and season.
Melt 25g butter and stir in 50g breadcrumbs for the topping.
Spread sauce over mushrooms and top with breadcrumbs and remaining cheese.
Bake for 15 minutes at 200°C until browned.