Season each fish fillet with 1 teaspoon of coriander, salt and pepper to taste, then using your preferred method of cooking, cook fish just until opaque throughout. Hold aside until needed.
Preheat oven to 400°F.
Lay out the 8 slices of bread and spread ½ ozs. of 1000 island dressing on each slice. Cut each piece of cheese on the bias, and place 2 of the cut slices on each piece of bread, covering as best you can.
Place one piece of fish on each of 4 pieces of bread. Top the fish with ½ cup of sauerkraut. Take the remaining 4 pieces of bread and close the sandwich, cheese down onto sauerkraut.
Heat a large oven-proof sauté pan. Butter top side of bread with ½ Tablespoon of softened butter, place butter side down into hot pan. Griddle until golden brown.
Butter exposed side of bread, carefully flip sandwich and place pan into oven.
Cook 5 minutes, or until cheese is melted and sandwich is heated through. Remove, cut each sandwich on the bias and place on serving plates. Serve with sides of your choosing.