In large skillet, heat oil over medium-high heat; add onions. Cook 5 minutes, stirring occasionally.
Reduce heat to medium-low; continue cooking onions 15 minutes or until browned and very tender, stirring frequently. Stir in rosemary, salt and pepper; remove from heat.
Spread bread slices evenly with mustard; top with Swiss cheese. Divide onion mixture evenly over cheese.
Place on foil-lined baking sheet; bake at 425F 10 minutes or until cheese is melted.
12 appetizers.
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