Heat 1 tablespoon of butter in a small skillet over medium heat and cook onions, stirring often and reducing temperature to low until onions are a deep golden brown and very soft, 15-20 minutes. Set aside.
Patties and Assembly:
Slice Turnbuckles in half, butter and top with White Truffle Salt, then bake following our heating instructions.
In a medium bowl, gently mix ground beef, salt and pepper. Form into 6 oval-shaped patties
Warm up your grill, if using a propane grill heat to 400 degrees, and cook patties for 5-8 minutes (depending on how well done you prefer). Transfer to a plate.
Assemble patty melts: Spread truffle mustard on bottom half of the Turnbuckle, top with a slice of Swiss cheese, caramelized onions, patties and more Swiss Cheese. Top with top of Turnbuckle to create sandwich.
Place patty melts back on grill and press with a heavy, grill-friendly object, like a cast-iron press or pan. Let cook until cheese is melted.