Put the cornmeal in a bowl with 1 cup of the water.

Bring the remaining 2 cups of water to a boil, then stir in the cornmeal. Stir often for about 7 minutes, until the cornmeal mush is thick and smooth.

Preheat the oven to 400°F. Grease an 8 x 8 x 2-inch baking dish.

Spread the cornmeal in the baking dish. Stir the vinegar and 1 teaspoon salt into the chopped spinach. Spread the spinach over the cornmeal and put the fish in a layer on top. Salt and liberally pepper the fish.

Bake in the oven for about 10 minutes, or until the center of the thickest part of the fish is opaque like the outside. Serve hot.