Fish and Mashed Potato Casserole

Peel and quarter potatoes, then cook in a small pot of boiling salted water, covered, until knife-tender, about 20 minutes.

Meanwhile, melt half of the butter in a large pot and add the flour, whisking constantly, and cook briefly to make a roux.

Whisking constantly, slowly add 550 ml (approximately 2 1/4 cups) of milk and simmer over low heat for 15 minutes.

Drain the potatoes, return to pot and let steam evaporate off the heat. 

Mash potatoes with a potato masher. Add remaining butter and stir in milk. Season with salt and pepper.

Rinse dill, shake dry, pluck fronds, chop finely and stir into the milk mixture in pot.

Remove any bones from both types of fish, then rinse, pat dry and cut into about 3 cm (approximately 1-inch) cubes. Season with salt and pepper.

Distribute fish in a baking dish, spread with dill sauce and then top with mashed potatoes.