Combine the mayonnaise, parsley, and dried oregano in a small bow and set it aside.
Butter the outside of the slices of sourdough bread.
Assemble the sandwich by layering the herbed mayo, caramelized onions, gruyere, arugula, and gouda between the non-buttered sides of the bread.
Place the sandwich buttered side down in a non-stick frying pan and place over medium-low heat. Cook for 6-7 minutes on each side until golden brown and crispy.
Serve alongside cherry tomatoes or a simple salad, and enjoy!