Clean and microwave potatoes for at least 5 minute (this makes them easier to handle, and then takes them less time to cook).
Wait 5 min after heating and cut potatoes into 1in cubes, set aside.
slice onion, set aside.
in medium bowl melt butter, mix in dijon mustard, parsley. garlic salt, onions and potatoes, makes sure they are evenly coated.
make an aluminum foil pouch, pour in potatoe mixture, seal pouch by rolling sides together (you may place this pouch into a small disposable aluminum pan so they dont leak).
heat grill on medium heat, place potatoes on grill cook for about 20-25 minutes (you may need more time), opening pouch and stirring mixture occansionally, i usually let mine go for a couple minutes more because i like when they get a little crispy.