Why not? After all, they have the onions. The story even answers the question that first popped into our head: if you use onions to run your onion factory, mightn’t you run out of onions to send around to customers?
One of Gill’s best-selling products is a line of sliced and diced fresh-cut onions. Because about 40 percent of each onion is unused in the process, the company generates some 150 tons of waste a day.
Okay, so they have the onions (though that seems like a lot of unused onion). Then what?
The system takes methane from fermented onion juice and converts it to energy that is burned in two fuel cells on-site.
Producing biofuel for a single company's closed-loop system is one thing, but integrating the energy into the public grid is still a prohibitively expensive and difficult endeavor.
Trying to scale this up just wouldn’t be very fun, plus people actually like onions on their hot dogs. Coal and uranium owe part of their energy exclusivity to making a terrible cereal (although not as terrible as wheat germ, we’d guess).
Gills has a page about this here.
Neither a leek nor a scallion.