Fish and Potato Casserole

Step 1
Preheat the oven to 350 degrees F (175 degrees C). Grease a 12×7-inch casserole dish.

Step 2
Slice potatoes 1/8 inch thick and place on a microwave-safe plate. Microwave on high until barely soft, 3 to 5 minutes; set aside.

Step 3
Melt butter in a saucepan over medium-low heat. Whisk in flour and stir until the mixture becomes paste-like and light golden brown, 2 to 4 minutes. Increase heat to medium and gradually whisk fish stock and milk. Bring to a simmer over medium heat. Cook and stir until the mixture is thick and smooth, about 10 minutes. Stir in 1/2 of the Cheddar cheese, nutmeg, and pepper. Remove from the heat.

Step 4
Layer 1/2 of the potatoes, 1/2 of the onion, and 1/2 of the spinach in the prepared casserole. Follow with white fish fillets and flaked salmon, then remaining onions, spinach, and potatoes. Pour all of the white sauce over top and sprinkle with Parmesan and remaining Cheddar.

Step 5
Bake uncovered in the preheated oven until center is hot and white fish flakes easily with a fork, about 40 minutes. Turn on the broiler and broil until lightly browned, 2 to 3 minutes.