Toast pecans in a dry skillet over medium-low heat for 3 to 4 minutes until fragrant, stirring often. Transfer to a plate and cool for 3 minutes.
Mix yogurt, mayonnaise, Dijon, lemon juice, salt and pepper in a bowl until smooth.
Fold in chopped turkey, apple, celery and cooled pecans until lightly coated without crushing the meat.
Place dry lettuce on 2 bread slices and divide turkey salad evenly over it, spreading close to the edges in a level layer.
Close with remaining bread, press gently and cut each sandwich in half before serving cold.