• 2 tablespoons olive oil
  • 2 lbs onions , thinly sliced
  • 1 1/2 tablespoons red wine vinegar
  • 1 tablespoon dark brown sugar , packed
  • 1/2 teaspoon caraway seed
  • 3 smoked sausage, fully cooked, such as white bratwurst or 3 chicken-apple sausages
  • 4 slices rye bread
  • 4 teaspoons Dijon mustard
  • 4 ounces gruyere cheese , thinly sliced
  • 3 tablespoons butter , room temperature
  • 1 Heat 2 tablespoons oil in heavy large skillet over medium-high heat.
  • 2 Add onions and sauté until slightly softened, about 5 minutes.
  • 3 Stir in vinegar, sugar and caraway seeds.
  • 4 Reduce heat to medium-low and sauté until onions are deep brown and very tender, about 20 minutes longer.
  • 5 Season to taste with salt and pepper.
  • 6 (Can be prepared 2 hours ahead. Let stand at room temperature. Rewarm over low heat before using.).
  • 7 Preheat broiler.
  • 8 Brush 11×7-inch metal baking pan with oil.
  • 9 Place sausages in pan.
  • 10 Broil until sausages are brown and crisp, about 6 minutes per side.
  • 11 Transfer sausages to work surface and cut lengthwise in half.
  • 12 Arrange 3 sausage halves on each of 2 bread slices.
  • 13 Spread 2 teaspoons mustard over sausages on each.
  • 14 Top sausages with caramelized onions, dividing equally.
  • 15 Top with Gruyère or Emmental cheese slices, dividing equally.
  • 16 Cover with remaining 2 bread slices.
  • 17 Spread half of butter over upper bread slices.
  • 18 Heat large nonstick skillet over medium-low heat.
  • 19 Carefully invert sandwiches, buttered side down, into skillet.
  • 20 Cover and cook until bottoms are brown, about 5 minutes.
  • 21 Spread remaining butter over upper bread slices.
  • 22 Turn sandwiches, cover and cook until bottoms are brown, about 4 minutes longer.