Preheat grill to medium-high heat (about 425 F).
In 12-inch skillet or pan over direct heat, caramelize onions in butter 15-20 minutes. Set aside.
In same pan, cook sliced bratwursts until browned and cooked through, 7-10 minutes. Set aside.
In same pan, spread drained German potato salad. Cook until browned, 5-7 minutes. Set aside.
Brush bottom of flatbread with olive oil.
Place flatbread directly on grill grates. Layer with browned potato salad, cooked Bratwurst slices, caramelized onions and cheese.
Close grill lid and cook 8-10 minutes, or until cheese is melted and flatbread is heated through.
Garnish with chopped chives and serve with fresh lemon wedges.