How to Make It
Heat a nonstick skillet over medium heat. Add oil to pan. Arrange 4 bread slices in pan. Layer cheese, prosciutto, and chicken over slices.
Combine sour cream and mustard in a bowl; spread over remaining 4 slices. Top sandwiches in pan with bread slices, sour cream side down.
Cook 3 minutes. Turn over; cook 2 to 3 minutes. Transfer to a cutting board. Spread butter evenly over tops of sandwiches; sprinkle evenly with thyme.