Prepare 8 pieces of aluminum foil with vegetable cooking spray. Combine potatoes, green onion, soup, cheese, (and bacon and mushrooms if using) in a bowl. Season to taste with salt and pepper. Evenly divide the potato mixture among the squares of aluminum foil. Dot with butter. Fold into foil packets, sealing well.
Cook foil packets on the grill for 12 to 20 minutes, until the potatoes have cooked.