Prepare two foil packets by layering together 4 squares of heavy duty aluminum foil for each packet. If using regular duty foil, use twice the number of layers. Spray the top sheet with nonstick cooking spray. In a bowl or resealable bag, toss together the potato slices, onion, and frozen vegetables; season to taste with salt, pepper, and garlic salt if using.
Evenly divide the potatoes between the two foil packets, and top with the divided butter. Fold each packet, starting length wise, rolling edges together, repeat for each end, making sure to seal edges tightly.
Cook the foil packets over the preheated grill until the potatoes are tender, approximately 15 minutes per side.