In a skillet, melt two tablespoons and add onions. Stirring the onions until caramelized, ten minutes, set aside.
Spread the remaining butter on each slice of bread. In the same skillet on low heat, place each slice of bread in the pan. Top each slice of bread with a slice of cheese and turkey.
Once cheese begins to melt, top with caramelized onions. Top with remaining bread slices and gently press together.