Spread butter on one side of bread slices. Mix jam and mustard; spread on non-buttered sides. Top 4 bread slices (on mustardy sides) with 2 slices cheese, peach, hazelnuts and remaining 2 slices cheese. Cap with remaining bread; buttered sides out.
Place sandwiches on grill preheated to medium. Cook, pressing down gently with spatula, 2 to 3 min. per side or until toasted and grill-marked. Turn off burner(s) directly under sandwiches and finish cooking over indirect heat for additional 3 to 5 min., or until cheese is melted.
Try grilling fruit and cheese combos such as apple and Cheddar, or provolone and pear.