Place the bread on a cutting board and spread the mayonnaise on both sides of each slice. On one slice, spread the Dijon, then top with the Gruyere, andSliced Jambon de Parisand caramelized onions. Place the other slice of mayo-coated bread on top
Place the sandwich in a medium nonstick skillet over medium heat and cook, occasionally pressing firmly with a spatula to smash, until golden on the bottom, about 4 minutes. Flip and continue to cook until the other side is golden and the cheese is completely melted, about 2 minutes more.
Transfer to a cutting board, slice and serve. Enjoy!
Pro Tip: Don’t rush it, medium heat allows the cheese to melt without burning the exterior!