COMBINE cranberry relish, mustard and sugar in small bowl; set aside.
ASSEMBLE sandwiches: Butter one side of each slice of bread. Place 4 slices, buttered side down, on a work surface. On each slice, spread about 2 tbsp. of the mustard mixture. Layer each with 1/2 cup turkey, 1/4 cup arugula and 1/4 cup French Fried Onions. Top each sandwich with 1 slice of bread, buttered side up.
HEAT panini or electric grill pan on medium-high. Cook sandwiches on covered grill until bread is toasted, about 8 min. Serve warm.