Solid! Sort of like you treat mint (I adore mint and have never had it take over the garden). Will research if they can be planted in an oak half-barrel or a fabric bag or something.
If you put one of those Egyptian onions in a fabric grow bag and sat it on the ground, the onions would grow to the ground and root in that ground and keep going across the ground.
If you put the bag on a deck the onion would grow onto the deck but it couldn’t root there, so maybe the plant would stay that size since it couldn’t walk away and root however you wouldn’t have any more onions since it couldn’t walk and root.
The best place is it’s own area that it can’t get out of. Let’s say you plant it in your front yard - eventually your whole front yard would be onions.
“These onions were made for walking...” instead of “These boots were made for walking...” song.