Grind together 1.5 tbsp chopped garlic, 1 tbsp lemon juice, 1 tbsp olive oil and 1/2 tsp each salt and pepper till it becomes a smooth paste.
Clean the fish fillet. Pat dry and make some gashes on it. Add the above marinade and marinate it for half an hour.
Cook the potatoes till done. Mash the cooked potatoes.
Add 1/2 tsp dried herbs, 2 tbsp butter, 1/4 cup hot milk, salt and pepper to the mashed potatoes. Mix well, until everything is combined. (refer notes)
To make white sauce: melt 30 gms butter in a wide pan. Add flour and cook till the raw smell of the flour goes, around 5-7 mins.
Add milk gradually while stirring the sauce. If it gets lumpy use a whisk (refer notes). Once the milk is added and begins to thicken, add cheese (2 tbsp), salt, pepper and dried herbs. Mix well and keep it aside.
Cook the marinated fish until it's done. Salmon tends to cook faster (3-4 mins on each side and it should be done).
Heat oil in the same pan in which you cooked fish. Add chopped onion and garlic (1.5 tsp). Once the onion browns a bit, add cooked mushroom and shredded salmon. Mix well. Add 1/4 - 1/3 cup of the prepared white sauce to the fish mixture. Mix well and remove from fire.
Grease an eight inch baking dish generously with butter. Transfer the fish mixture to the baking dish. Top it with the remaining white sauce.
Spread the mashed potatoes on top of the white sauce. Sprinkle the grated cheese as the top layer. Bake in the preheated oven for 20 - 25 mins. During the last 5 mins, put it on broil to get a golden crust.
Let the pie rest for 20 - 25 mins, before serving. This helps the pie to set and it will be easier to serve. Serve with garlic bread.
Notes
You can use any fish fillet with meaty flesh. The cooking time may vary depending on the type of fish you use. I used skinless fillet. If using fillet with skin, remove the skin before shredding the fish. If you dont have dried herbs, you can use fresh parsley. I used gruyere cheese, you can use cheddar or any other melting cheese.The texture of mashed potatoes can be smooth or a bit lumpy according to your preference.I also added a handful of baby spinach along with mushrooms.If the white sauce is still lumpy even after using a whisk, pass it through a sieve and it should be smooth.