How to Make It
Spread whole-grain mustard on one slice of bread and top with 1/3 cup Gruyère cheese, turkey, carmelized sweet onions, and remaining 1/3 cup Gruyère cheese, in that order. Top with remaining slice of bread. Melt butter in a small skillet over medium until beginning to foam. Add sandwich to skillet and cook until first side browned, about 2 minutes. Flip sandwich and cover pan with aluminum foil. Cook until cheese is melted and second side is browned, about 2 more minutes. Serve immediately.