Prepare your gas or charcoal grill to high heat.
In a large bowl, toss together the cubed potatoes, chopped onion, olive oil and garlic salt. Break up the cooked bacon and toss small bits into the potato mixture, making sure everything is evenly distributed.
Tear 4 large sheets of aluminum foil, about 16 inches long each. Divide the potato mixture amongst the 4 sheets of aluminum foil and fold the sides up on both ends so you create a securely wrapped parcel for the potatoes.
Place the 4 parcels on the grill and cover. Let the potatoes cook for about 7-8 minutes on each side. When flipping the potato parcels, be sure to use a pair of tongs and a spatula so the parcels stay intact.
Once the potatoes are fully cooked and fork tender, you can peek into the parcel and check, open up each of the parcels and distribute the cheese on top of each portion of potatoes along with the chopped scallions. Let the parcels cook for about 1 minute more just to melt the cheese.
Carefully remove the potato parcels from the grill and serve the potatoes either in the aluminum foil or transfer them to individual plates. Season with salt and pepper as needed.