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. 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 on top of the batter. Pour in the rest of the batter.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, about 30 minutes.
