BEST RUSSIAN FISH AU GRATIN

INGREDIENTS
300g fish fillet (tilapia, bass, or catfish), halved
1 tablespoon lemon juice
1 teaspoon salt
1 teaspoon black pepper
2 teaspoon vegetable oil
1 cup mushroom, sliced
1 teaspoon whiskey (option)
1/4 onion, sliced
150ml heavy cream
100ml milk
1/2 teaspoon dried parsley
1/2 teaspoon dried basil
1/2 teaspoon white pepper
1/2 teaspoon salt
2 egg yolks
3 tablespoon flour
2 egg whites
2 tablespoon butter
1 small potato, sliced
2 handful grated mozzarella cheese
1 tablespoon grated parmesan cheese


INSTRUCTIONS
Dry fish with paper towel and place on a plate. Add lemon juice, rubbed salt and black pepper on fish, marinate in the fridge for 20-30 minutes.
Heat a teaspoon oil in a non-sticky skillet till hot, in about 1 minute, add mushrooms and stir fry till turn soft, add salt, pepper and whiskey, continue to stir fry till all the liquid evaporate and mushrooms turn golden. Put mushrooms in a plate and set aside.
Heat rest of oil in the same skillet and add onion, stir fry till onion turn soft and golden. Put in a plate and set aside.
Add heavy cream and milk into a medium size bowl, add parsley, basil, white pepper, salt and egg yolks, stir till everything mix well. (* see footnote)
Preheat oven till 180 degres C (350F).
Place flour in a big plate and egg whites in another big plate. Heat 1 tablespoon butter in a non-sticky skillet and heat on medium high heat. Coat each side of fish fillet with flour. When butter is melted and stop bubbling, dip both sides of fish fillet in egg whites, and place in the skillet immediately. Fry fish till the bottom side turns golden, then flip to fry the other side. When both sides are done, remove skillet from heat. Do not cook fish for too long time, since it will continue to cook in the oven.
Grease 2 medium size baking dishes with the rest butter. Place half amount of onion in both dishes separately, add fish on top of onion, place potato slices against the wall of dish. Add the rest onion on top of fish, then place mushrooms on onion. Fill each dish with the milk mixture, around 2/3 of the dish or 1cm to the top. Top with a thin layer of mozzarella cheese, then sprinkle with parmesan cheese.
Place the baking dishes on baking tray, put in the bottom level of oven, bake till the cheese melts well and turns golden brown, in about 30 minutes. If cheese does not brown well, switch baking tray to middle level at the end of baking process.
NOTES
The amount of sauce in this recipe is quite small, because I don't have big baking dish. If you use a bigger or deeper baking dish/baking pan, double or triple the ingredients of sauce. The sauce should be able to cover the fish and all the vegetables.