Make a foil boat with heavy duty aluminum foil with approximately a 3" x 5" bottom surface and enough extra on sides to seal the top, (see below).
Peel and slice onions at least 1/4 inch thick, (can go thicker). Spread margarine on the bottom, separate and add onion slices, (overlapping is okay). Sprinkle with garlic salt to taste and fold excess foil together, (like you are folding down the top of a paper lunch bag).
Wait until the last 15 minutes of steak cook time and add to grill, (bottom rack). Cook for the 15 minutes or to desired taste. Serve as a side dish for steak or any beef such as burgers or a roast, or serve on top of steaks.