Peel, wash, and chop the potatoes. Cover with water put on fire. Bring to a boil, season with salt, and simmer, covered with a low boil for 30 minutes.
Then drain the water, add butter and mash the hot potatoes into mashed potatoes. Cool down.
Add egg, ground black pepper, and nutmeg. Continue to knead with a crush and puree. Continuing to mash potatoes, add flour. Stir until smooth.
Turn on the oven to preheat. Prepare the filling: First, grate the cheese. Finely chop the fillet. Then mix 50 g of cheese, salmon fillet, and green peas.
Take a form about 20 x 15 cm in size. Put the puree in an even layer, covering the bottom and sides. Place the filling in the middle of the casserole. Top with the remaining grated cheese.
Bake the potato casserole with fish for 20 minutes in an oven preheated to 190-200 degrees. The casserole is ready when a golden-brown crust forms on top.