Place a cast iron frying pan in your grill to preheat while preparing the bread and fish.
Flatten the sliced bread with a rolling pin until it is about 1/8 inch thick.
Trim fish filets so that the pieces are small enough to fit on a slice of rolled bread. You can cut them smaller if desired.
Place a piece of fish on a piece of bread and using a sharp point knife, cut the bread along the outline of the piece of fish. Season the fish to taste with salt and pepper, a prepared seasoning, or whatever herbs and spices you prefer on fish. Spread some butter on the exposed side of the bread and place into the frying pan which should be medium hot. Allow the fish to cook much the same as you would a grilled cheese sandwich. Turn the fish over when the bread has browned and cook until the fish is done. This should take only a few minutes per side.