1) Preheat a griddle pan to high heat.
2) Add sliced onion to griddle pan and cook until tender, about 10 minutes.
3) In a medium bowl, mix minced beef, onion powder, salt and freshly ground pepper. Make burgers equal in size, about 5cm across.
4) Cook burgers on the flat top for 3-4 minutes on each side.
Serve on split rolls with griddled onions and mustard sauce
For the mustard sauce:
Mix all ingredients in a mixing bowl