Place sliced potatoes with onions and garlic on top in a microwave dish - a plate, pie plate or whatever is a safe microwavable dish. Cover with a paper towel. Cook on high 20 minutes. (Your cooking time really depends a lot on how thick you cut your potatoes, you really want them about halfway done.) Rearrange potatoes on plate and cook about 7 more minutes. I use 2 long pieces of aluminum foil, enough to make a nice packet to hold the potatoes. I lay one lengthwise and one crosswise of that one. Spray with cooking spray. Place sliced potatoes on foil, put onions on top of potatoes, season with a few pats of butter, Mrs. Dash, salt and pepper. Fold foil over potatoes making a nice tight packet. Place on top rack of grill and finish cooking while you are cooking your meat, about 20 - 25 minutes, there's no need to turn. The potatoes are actually steaming in the packet. Remove from grill and while still hot, top with more butter and shredded cheese, add other toppings as desired.