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