Wash spinach, and remove excess water. Soak butterfish in water for about 2 hours. Drain, and cut fish into 1/4-inch slices.
In a 9 x 13-inch pan, layer in the following order: 1 pound spinach, pork cubes, sprinkle of Hawaiian salt, butterfish, and remaining spinach. Seal tightly with foil, and bake at 350 degrees F for 1-1/2 hours. Cut into 12 portions, and serve immediately with poi or hot rice.