Combine the olive oil, garlic, and rosemary in a microwave-safe bowl. Place a napkin on top and heat at 30 second intervals until hot and flavors are blended, approximately 1 to 2 minutes.
Slice the bread into 1-1/2 ” slices without cutting all the way through the bottom crust, so that the loaf remains intact. Drizzle the garlic herb olive oil into each cut. Sprinkle the cheese into each cut. Wrap the bread loaf completely in foil and place on grill to the side of the fish until the bread is heated through and cheese is melted. Turn frequently to avoid burning.