Heat a large skillet or griddle until hot. Butter one side of each slice of bread. Place four slices of bread in pan, butter-side down. Top each with one slice of Fontina, equal portion of sauteed greens, and another slice of Fontina. Cover each with remaining slices of bread, butter-side up.
Cook until bottom side of bread is browned and cheese is melting (reduce heat if bread starts browning too quickly). Flip each sandwich over and cook until other sides are browned and all cheese is melted. Cut and serve.