Preheat oven to 375°F (190°C).

Cook macaroni in boiling water for 10 minutes.

Drain the cooked macaroni and set aside.

Drain the liquid from the can of fish into a 1-pint cup.

Add enough milk to the fish liquid to make 2 cups.

Flake the fish into smaller pieces.

Melt butter or margarine in a heavy saucepan.

Stir in flour, seasoned salt, instant onion, pepper, and dry mustard.

Remove the saucepan from the heat.

Stir in the milk mixture until well combined.

Return the saucepan to the heat and cook, stirring constantly, until the sauce has thickened.

Add 1 cup of shredded Cheddar cheese to the sauce and stir until melted and blended.

Fold in the cooked pasta, flaked fish, Worcestershire sauce (if using), sour cream, and peas into the cheese sauce.

Pour the mixture into a greased baking pan.

Sprinkle the remaining 1/4 cup of crushed saltines over the top of the casserole.

Bake in the preheated oven for 40 minutes, or until golden brown and bubbly.