Another tip from McMenu - McDonald's Do-It-Yourself Recipes:
The regular Hamburgers, the Big Mac, and the Breakfast Burrito all use reconstituted onions. You can buy these at your local supermarket in the spice aisle. Just look for dried chopped onion. Make sure they are chopped onion, and not minced. To re-hydrate or reconstitute these dried onions, simply add 4 parts water to one part dried onion, cover, and refrigerate 30 minutes. After 30 minutes, drain the liquid and BAM - there they are! McDonald's little baby onions. Use the proper amount needed for your recipe, and refrigerate (covered) the leftovers.