Step 1

Preheat the oven to 350 degrees F.

Step 2

Bring a large pot of water to a boil.

Step 3

Add the cubed potatoes to the boiling water and cook until the potatoes are fork tender, about 15-20 minutes.

Step 4

Drain the potatoes. Place the drained potatoes back into the pot or into a large mixing bowl.

Step 5

Add the 3 tablespoons of the butter, 2/3 cup of the milk, and 1/2 teaspoon of the salt to the potatoes and mash until creamy.

Step 6

In a large skillet, melt the remaining butter over medium heat.

Step 7

Add the green onions, the carrots, and the garlic and cook for 2 minutes.

Step 8

Sprinkle the flour over the veggie mixture and stir, cooking until the raw flour smell disappears, about 2 minutes.

Step 9

Slowly pour the remaining milk into the skillet, whisking constantly until smooth and thickened, about 3 minutes.

Step 10

Add 1 cup of the cheddar cheese, the mustard, and the dill to the sauce and stir until the cheese melts.

Step 11

Add in the fish, gently stirring and cooking until beginning to warm, about 1-2 minutes.

Step 12

Stir the peas into the fish mixture. Season with the remaining salt and pepper.

Step 13

Pour the filling into a 9x13-inch ovenproof dish.

Step 14

Top the fish filling with the mashed potatoes, smoothing it out with a spoon and roughing up the top with a fork.

Step 15

Sprinkle the top of the mashed potatoes with the remaining cheddar cheese.

Step 16

Place the dish on a baking pan and bake until the top starts to golden, about 30-35 minutes. Sprinkle the top with a little chopped dill. Serve.