Preheat oven to 400 degrees F (200 degrees C).

Butter a cast iron skillet or pie plate.

Mix sour cream and baking soda together in a large bowl.

Add 1 cup plus 3 tablespoons flour, eggs, mayonnaise, and salt to the sour cream mixture.

Mix to form a thin batter of pouring consistency.

Pour 1/2 the batter into the prepared skillet or pie plate.

Mash fish with a fork.

Spread mashed fish on top of the batter in the skillet.

Pour the remaining batter over the fish.

Bake in the preheated oven until a toothpick inserted into the center comes out clean, about 30 minutes.