Chop onions, separating white portion from green. Place white portion in small saucepan; reserve green portion. Add sauce mix and milk to saucepan; beat with wire whisk to combine. Add butter; cook over medium heat until mixture comes to a boil, stirring constantly. Reduce heat to low; simmer 1 minute. Cover; refrigerate at least 2 hours or until thickened and cool.
Meanwhile, heat oven to 400°F. Place pork tenderloins in shallow roasting pan; rub surfaces with garlic salt and pepper. Bake at 25 to 30 minutes or until no longer pink in center and meat thermometer inserted in center registers 160°F. Cool 20 minutes or until cool enough to handle and slice.
Reduce oven temperature to 375°F. Place bread slices on ungreased cookie sheet. Bake 7 to 9 minutes or until crisp. Cool 10 minutes or until completely cooled.
To serve, stir reserved green portion of onions into sauce. Cut pork into 32 slices. Place 1 slice pork on each toasted bread slice, folding slice if necessary. Top each with 1 teaspoon sauce; reserve remainder for another use. Top each with 1/8 teaspoon crushed pink peppercorns.