Fisherman's Pie

Step 1
Preheat the oven to 375 degrees F (190 degrees C).

Step 2
Arrange cod in a single layer in a large baking dish; 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 5 minutes.

Step 4
Gradually whisk warm milk into the flour mixture and bring to a simmer over medium heat. Whisk until the sauce is thick and smooth, about 5 minutes.

Step 5
Whisk in white Cheddar cheese, then mix onion, spinach, salt, and black pepper into the sauce.

Step 6
Remove saucepan from heat and stir in garlic. If sauce is too thick, stir in more warm milk, 1 tablespoon at a time.

Step 7
Pour sauce over fish in baking dish.

Step 8
Layer mashed potatoes on top.

Step 9
Bake in the preheated oven for 30 minutes, then sprinkle with yellow Cheddar cheese. Continue baking until fish is easily flaked with a fork, 15 to 20 minutes more.