I do find that larger pieces of green onions do not really rehydrate well. They turn rather stringy and tough, so I find it's best to freeze the onions I want to mix into recipes for soups, salads, etc. and turn the dried onions into herb blends and powders to season with.
To dehydrate cut green onions into small pieces. (You can cut them smaller than I did!)
Dry on lowest setting (if you can adjust your temperature) for 4-5 hours till completely dried and the onions are easily crumbled.
From here you can puliverize into powder in a food processor or crush into small pieces to mix into a Savory Herb blend with dried herbs such as rosemary, thyme, oregano, sage
Store in cool, airtight container.