Rinse potatoes. Boil until tender. Drain.
Mix potatoes with milk, butter, sour cream, salt, and pepper.
Heat milk. Simmer fish and onion for 6-7 mins.
Sauté onion, leeks, garlic in butter and oil. Add flour.
Add poaching milk. Thicken. Add bouillon, cheese. Stir.
Spoon fish mixture into dish. Top with mash. Bake at 350℉ for 25-30 mins. Broil for golden brown.