03Arrange 3 sausage halves on each of 2 bread slices. Spread 2 teaspoons mustard over sausages on each. Top sausages with caramelized onions, dividing equally. Top with Gruyère or Emmenthal cheese slices, dividing equally. Cover with remaining 2 bread slices. Spread half of butter over upper bread slices.
04Heat large nonstick skillet over medium-low heat. Carefully invert sandwiches, buttered side down, into skillet. Cover and cook until bottoms are brown, about 5 minutes. Spread remaining butter over upper bread slices. Turn sandwiches, cover and cook until bottoms are brown, about 4 minutes longer.