Add the potatoes to a large pan and cover with water. Bring to a boil and simmer for 10-15 minutes until tender when a sharp knife is inserted. Drain and put into a bowl. Add butter, salt and pepper, mash and set aside.

Preheat oven to 400°F/200°C. Rub a 12 x 9 x 2-inch (31 x 23 x 6 cm) size casserole dish with butter.

To a large pan add the fish, milk, pearl onions, bay leaf and salt. Simmer for 5 minutes. Turn off the heat and allow fish to cool in the liquid. Once cooled, strain the milk through a sieve separating the fish and milk. Remove the bay leaf and set the fish and milk aside.

To a pan add the butter over medium heat. When melted and bubbling whisk in the flour. Slowly pour in the milk broth from the fish while whisking, simmer until thickened. Stir in the grated cheese until melted (if using). Add the fish and pearl onions, shrimp, peas, parsley and nutmeg. Taste for seasoning and add salt if needed.

Pour the fish and vegetables into a baking dish. Top with the mashed potato (you don't have to pipe it) and brush the top of the mash with melted butter. Place on a large baking sheet (in case it bubbles over).

Bake for 20-25 minutes until the top is golden brown. If the top doesn’t brown as much as you want it to, heat under the broiler (grill if you’re in the U.K.), but keep any eye on it until browned.