Step 1

Heat oven to 400°F.

Step 2

Melt butter in large skillet on medium heat. Add ham and onions; cook on medium-low heat 12 to 14 min. or until onions are tender and golden brown, stirring occasionally. Remove skillet from heat.

Step 3

Unfold pastry sheet; cut into 12 squares. Press 1 square onto bottom and up side of each of 12 muffin pan cups sprayed with cooking spray.

Step 4

Whisk eggs, sour cream and nutmeg in medium bowl until blended. Add onion mixture; mix well. Spoon into pastry cups; top with Swiss pieces. Sprinkle with chives.

Step 5

Bake 12 to 14 min. or until filling is set in centers, and edges of pastry cups are golden brown. Transfer tarts from muffin pan to wire racks; cool slightly.