1/4 - cup pure maple syrup
1 - Tablespoon extra virgin olive oil
1 - Tablespoon white wine vinegar
1 - teaspoon salt
2 - cups sweet onion, thinly sliced
8 - large slices rye bread
1/4 - cup margarine or butter, softened
1/4 - cup whole grain mustard or stone ground mustard
1/4 - pound Swiss cheese, thinly sliced
3/4 - pound thinly sliced maple mustard turkey breast

• In medium bowl, stir together the maple syrup, olive oil, vinegar and salt. 
• Add onion, toss to coat with syrup mixture. 
• Let stand 20 to 30 minutes at room temperature, stirring several times.
  
• Spread one side of bread with margarine. 
• Place 4 slices margarine side down. 
• Spread 1-tablespoon mustard over each of 4 slices of bread and cover with half the cheese.
  
• Scatter marinated onions over cheese and top with turkey. 
• Place remaining cheese over turkey and complete with remaining bread, buttered side up.
  
• Heat large heavy skillet over medium heat. 
• Add 1 to 2 sandwiches at a time, cover with lid and heat until bottom side turns golden brown. 
• Turn sandwich over and continue heating until browned.